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Hypersonics Before
the Shuttle
A Concise History of the
X-15 Research Airplane
Dennis R. Jenkins
Monographs in Aerospace History
Number 18
June 2000
NASA Publication SP-2000-4518
National Aeronautics and Space Administration
NASA Office of Policy and Plans
NASA History Office
NASA Headquarters
Washington, D.C. 20546
For sale by the U.S. Government Printing Office
Superintendent of Documents, Mail Stop: SSOP, Washington, DC 20402-9328
ISBN 0-16-050363-9
The use of trademarks or names of manufacturers in this
monograph is for accurate reporting and does not constitute
an official endorsement, either expressed or implied, of such
products or manufacturers by the National Aeronautics and
Space Administration.
Library of Congress Cataloging-in-Publication Data
Jenkins, Dennis R.
Hypersonics before the shuttle
A concise history of the X- 15 research airplane / by Dennis R. Jenkins
p. cm. ~ (Monographs in aerospace history ; no. 18) (NASA history series) (NASA
publication ; SP-2000-4518)
Includes index.
1. X-15 (Rocket aircraft)-History. I. Title. II. Series. HI. Series: NASA history series
IV. NASA SP ; 2000-4518.
TL789.8.U6 X553 2000
629.133'38-dc21
00-038683
Table of Contents
Preface Introduction and Author's Comments 4
Chapter 1 The Genesis of a Research Airplane 7
Chapter 2 X-15 Design and Development 21
Chapter 3 The Flight Research Program 45
Chapter 4 The Legacy of the X-15 67
Appendix 1 Resolution Adopted by NACA Committee on Aerodynamics 85
Appendix 2 Signing the Memorandum of Understanding 86
Appendix 3 Preliminary Outline Specification 92
Appendix 4 Surveying the Dry Lakes 102
Appendix 5 R&D Project Card— Project 1226 Ill
Appendix 6 X-15 Flight Designation System 115
Appendix 7 Major Michael J. Adams Joins the Program 116
Appendix 8 Astronaut Wings 117
Appendix 9 X-15 Program Flight Log 118
Index 122
One of the NB-52s
flies over the X-1 5-1
on Edwards Dry Lake
in September 1961.
(NASA photo
EC61-0034)
IV AS4L
Monographs in Aerospace History Number 1 8 — Hypersonics Before the Shuttle
Introduction and Author's Comments
Preface
Preface
Introduction and Author's Comments
It is a beginning. Over forty-five years have
elapsed since the X-15 was conceived; 40
since it first flew. And 31 since the program
ended. Although it is usually heralded as the
most productive flight research program ever
undertaken, no serious history has been
assembled to capture its design, develop-
ment, operations, and lessons. This mono-
graph is the first step towards that history.
Not that a great deal has not previously been
written about the X-15, because it has. But
most of it has been limited to specific aspects
of the program; pilot's stories, experiments,
lessons-learned, etc. But with the exception
of Robert S. Houston's history published by
the Wright Air Development Center in 1958;
and later included in the Air Force History
Office's Hypersonic Revolution, no one has
attempted to tell the entire story. And the
WADC history is taken entirely from the Air
Force perspective, with small mention of the
other contributors.
In 1954 the X-l series had just broken Mach
2.5. The aircraft that would become the X-15
was being designed to attain Mach 6, and to
fly at the edges of space. It would be accom-
plished without the use of digital computers,
video teleconferencing, the internet, or email.
It would, however, come at a terrible financial
cost — over 30 times the original estimate.
The X-15 would ultimately exceed all of its
original performance goals. Instead of Mach
6 and 250,000 feet, the program would
record Mach 6.7 and 354,200 feet. And com-
pared against other research (and even oper-
ational) aircraft of the era, the X-15 was
remarkably safe. Several pilots would get
banged up; Jack McKay seriously so,
although he would return from his injuries to
fly 22 more X-15 flights. Tragically, Major
Michael J. Adams would be killed on Flight
191, the only fatality of the program.
Unfortunately due to the absence of a subse-
quent hypersonic mission, aeronautical
applications of X-15 technology have been
few. Given the major advances in materials
and computer technology in the 30 years
since the end of the flight research program,
it is unlikely that many of the actual hard-
ware lessons are still applicable. That being
said, the lessons learned from hypersonic
modeling, simulation, and the insight gained
by being able to evaluate actual X-15 flight
research against wind tunnel and predicted
results, greatly expanded the confidence of
researchers. This allowed the development of
Space Shuttle to proceed much smoother
than would otherwise have been possible.
In space, however, the X-15 contributed to
both Apollo and Space Shuttle. It is interest-
ing to note that when the X-15 was con-
ceived, there were many that believed its
space-oriented aspects should be removed
from the program since human space travel
was postulated to be many decades in the
future. Perhaps the major contribution was
the final elimination of a spray-on ablator as
a possible thermal protection system for
Space Shuttle. This would likely have hap-
pened in any case as the ceramic tiles and
metal shingles were further developed, but
the operational problems encountered with
the (admittedly brief) experience on X-15A-2
hastened the departure of the ablators.
Many people assisted in the preparation of
this monograph. First and foremost are Betty
Love, Dill Hunley, and Pete Merlin at the
DFRC History Office. Part of this project
Dennis R. Jenkins is
an aerospace engi-
neer who spent
almost 20 years on
the Space Shuttle pro-
gram for various con-
tractors, and has also
spent time on other
projects such as the
X-33 technology
demonstrator.
He is also an author
who has written over
20 books on aero-
space history.
Hypersonics Before the Shuttle — Monographs in Aerospace History Number J 8
Preface
Introduction and Author's Comments
was assembling a detailed flight log (not part
of this monograph), and Betty spent many
long hours checking my data and researching
to fill holes. I am terribly indebted to her.
Correspondence continues with several of
the program principals — John V. Becker,
Scott Crossfield, Pete Knight, and William
Dana. Dr. Roger Launius and Steve Garber at
the NASA History Office, and Dr. Richard
Hallion, Fred Johnsen, Diana Cornelisse,
and Jack Weber all provided excellent sup-
port for the project. A. J. Lutz and Ray
Wagner at the San Diego Aerospace Museum
archives, Tony Landis, Brian Lockett, Jay
Miller, and Terry Panopalis also provided
tremendous assistance to the project.
Dennis R. Jenkins
Cape Canaveral, Florida
February 2000
With the XLR99
engine lagging behind
in its development
schedule, theX-15
program decided to
press ahead with ini-
tial flights using two
XLR11 engines— the
same basic engine
that had powered the
Bell X-1 on its first
supersonic flight. (San
Diego Aerospace
Museum Collection)
When the Reaction
Motors XLR99 engine
finally became avail-
able, the X-1 5 began
setting records that
would stand until the
advent of the Space
Shuttle. Unlike the
XLR1 1 , which was
"throttleable" by ignit-
ing different numbers
of thrust chambers,
the XLR99 was a truly
throttleable engine
that could tailor its
output for each specif-
ic mission. (San Diego
Aerospace Museum
Collection)
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
Hydraulic lifts were
installed in the ramp
at the Flight Research
Center (now the
Dryden Flight
Research Center) to
lift the X-15 up to the
wing pylon on the
NB-52 mothership.
(Jay Miller Collection)
fct>.
4 "t-» 1,B!S
The early test flights
were conducted with a
long air data probe
protruding from the
nose of the X-15.
Notice the technician
manually retracting
the nose landing gear
on the X-15, some-
thing accomplished
after the research air-
plane was firmly con-
nected to the wing of
the NB-52 mothership.
(San Diego
Aerospace Museum
Collection)
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 1
The Genesis of a Research Airplane
Chapter 1
The Genesis of a Research Airplane
It was not until the mid-1940s that it became
apparent to aerodynamic researchers in the
United States that it might be possible to build
a flight vehicle capable of hypersonic speeds.
Until that time, propulsion systems capable of
generating the thrust required for such vehi-
cles had simply not been considered techni-
cally feasible. The large rocket engines that
had been developed in Germany during World
War II allowed concept studies to be initiated
with some hope of success.
Nevertheless, in the immediate post-war peri-
od, most researchers believed that hypersonic
flight was a domain for unmanned missiles.
When an English translation of a technical
paper by German scientists Eugen Sanger and
Irene Bredt was provided by the U.S. Navy's
Bureau of Aeronautics (BuAer) in 1946, this
preconception began to change. Expanding
upon ideas conceived as early as 1928, Sanger
and Bredt had concluded during 1944 that a
rocket-powered hypersonic aircraft could be
built with only minor advances in technology.
The concept of manned aircraft flying at
hypersonic speeds was highly stimulating
to researchers at the National Advisory
Committee for Aeronautics (NACA). 1 But
although there were numerous paper studies
exploring variations of the Sanger and Bredt
proposal in the late 1940s, none bore fruit and
no hardware construction was undertaken at
that time. It was from this background, how-
ever, that the concept for a hypersonic
research airplane would emerge. 2
At the time, there was no established need for
a hypersonic aircraft, and it was assumed by
many that no operational military 3 or civil
requirement for hypersonic vehicles would be
forthcoming in the foreseeable future. The
need for hypersonic research was not over-
whelming, but there was a growing body of
opinion that it should be undertaken.
The first substantial official support for hyper-
sonic research came on 24 June 1952 when the
NACA Committee on Aerodynamics passed a
resolution to ". . . increase its program dealing
with the problems of unmanned and manned
flight in the upper stratosphere at altitudes
between 12 and 50 miles, 4 and at Mach num-
bers between 4 and 10." This resolution was
ratified by the NACA Executive Committee
when it met the following month. A study
group consisting of Clinton E. Brown (chair-
man), William J. O' Sullivan, Jr., and Charles
H. Zimmerman was formed on 8 September
1952 at the Langley 5 Aeronautical Laboratory.
This group endorsed the feasibility of hyper-
sonic flight and identified structural heating as
the single most important technological prob-
lem remaining to be solved.
An October 1953 meeting of the Air Force's
Scientific Advisory Board (SAB) Aircraft
Panel provided additional support for hyper-
sonic research. Chairman Clarke Millikan
released a statement declaring that the feasi-
bility of an advanced manned research aircraft
"should be looked into." The panel member
from Langley, Robert R. Gilruth, played an
important role in coordinating a consensus of
opinion between the SAB and the NACA.
Contrary to Sanger's conclusions, by 1954 it
was generally agreed within the NACA and
industry that the potential of hypersonic flight
could not be realized without major advances
in technology. In particular, the unprecedent-
ed problems of aerodynamic heating and
high-temperature structures appeared to be
so formidable that they were viewed as
"barriers" to sustained hypersonic flight.
Monographs in Aerospace History Number 18 — Hypersordcs Before the Shuttle
The Genesis of a Research Airplane
Chapter 1
Fortunately, the successes enjoyed by the sec-
ond generation X-ls and other high-speed
research programs had increased political and
philosophical support for a more advanced
research aircraft program. The. large rocket
engines being developed by the long-range
missile (ICBM) programs were seen as a way
to provide power for a hypersonic research
vehicle. It was now agreed that manned
hypersonic flight was feasible. Fortunately, at
the time there was less emphasis than now on
establishing operational requirements prior to
conducting basic research, and perhaps even
more fortunately, there were no large manned
space programs with which to compete for
funding. The time was finally right for launch-
ing a hypersonic flight research program. 6
The specific origins of the hypersonic
research program occurred during a meeting
of the NACA inter-laboratory Research
Airplane Panel held in Washington, DC, on 4-
5 February 1954. The panel chairman, Hartley
A. Soule, had directed NACA research air-
craft activities in the cooperative USAF-
NACA program since 1946 and was well
versed in the politics and personalities
involved. The panel concluded that a wholly
new manned research vehicle was needed,
and recommended that NACA Headquarters
request detailed goals and requirements for
such a vehicle from the research laboratories.
In responding to the NACA Headquarters, all
of the NACA laboratories set up small ad hoc
study groups during March 1954. Langley
had been an island of hypersonic study since
the end of the war and chose to deal with the
problem in more depth than the other labora-
tories. After the new 1 1-inch hypersonic wind
tunnel at Langley became operational in 1947,
a research group headed by Charles H.
McLellan was formed to conduct limited
hypersonic research. 7 This group, which
reported to the Chief of the Langley Aero-
Physics Division, John V. Becker, provided
verification of newly developed hypersonic
theories while investigating such important
phenomena as hypersonic shock-boundary-
layer interaction. The 11 -inch tunnel later
served to test preliminary design configura-
tions that led to the final hypersonic aircraft
configuration. Langley also organized a paral-
lel exploratory program into materials and
structures optimized for hypersonic flight.
Given this, it was not surprising that a team at
Langley was largely responsible for defining
the early requirements for the new research
airplane. The members of the Langley team
included Maxim A. Faget in propulsion;
Thomas A. Toll in configuration, stability, and
control; Norris F. Dow in structures and mate-
rials; and James B. Whitten in piloting. All
four fell under the direction of Becker. Besides
the almost mandatory elements of stability,
control, and piloting, a fourth objective was
outlined that would come to dominate virtual-
ly every other aspect of the aircraft's design —
it would be optimized for research into the
related fields of high-temperature aerodynam-
ics and high-temperature structures. Thus it
would become the first aircraft in which aero-
thermo-structural considerations constituted
the primary research problem, as well as the
primary research objective.
The preliminary specifications for the
research aircraft were surprisingly brief: only
four pages of requirements, plus six addition-
al pages of supporting data. A new sense of
urgency was present: "As the need for the
exploratory data is acute because of the rapid
advance of the performance of service air-
craft, the minimum practical and reliable air-
plane is required in order that the develop-
ment and construction time be kept to a mini-
mum." 8 In other versions of the requirements
this was made even more specific: "It shall be
possible to design and construct the airplane
within 3 years." 9 As John Becker subsequent-
ly observed, ". . . it was obviously impossible
that the proposed aircraft be in any sense an
optimum hypersonic configuration."
In developing the general requirements, the
team developed a conceptual research aircraft
that served as a model for the eventual X-15.
The aircraft they conceived was "... not pro-
posed as a prototype of any of the particular
8
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 1
The Genesis of a Research Airplane
The first Bell X-2
(46-674) made its ini-
tial unpowered glide
flight on 5 August
1 954. This aircraft
made a total of 1 7
flights before it was
lost on 27 September
1956. Its pilot, Air
Force Captain Milburn
Apt had flown to a
record speed 2,094
mph, thereby becom-
ing the first person to
exceed Mach 3.
(NASA/DFRC)
concepts in vogue in 1954 . . . [but] rather as a
general tool for manned hypersonic flight
research, able to penetrate the new regime
briefly, safely, and without the burdens,
restrictions, and delays imposed by opera-
tional requirements other than research." The
merits of this approach had been convincing-
ly demonstrated by the successes of the X-l
and other dedicated research aircraft of the
late 1940s and early 1950s. 10
Assuming that the new vehicle would be air
launched like the X- 1 and X-2, Langley estab-
lished an aircraft size that could conveniently
be carried by a Convair B-36, the largest suit-
able aircraft available in the inventory. This
translated to a gross weight of approximately
30,000 pounds, including 18,000 pounds of
fuel and instrumentation. 11 A maximum speed
of 4,600 mph and an altitude potential of
400,000 feet were envisioned, with the pilot
subjected to approximately 4.5g (an accelera-
tion equal to 4.5 times the force of gravity) at
engine burnout. 12
The proposed maximum speed was more than
double that achieved by the X-2, and placed
the aircraft in a region where heating was the
primary problem associated with structural
design, and where very little background
information existed. Hypersonic aerodynam-
ics was in its infancy in 1954. The few small
hypersonic wind tunnels then in existence had
been used almost exclusively for fluid
mechanics studies, and they were unable to
simulate either the high temperatures or the
high Reynolds numbers of actual flight. It was
generally believed that these wind tunnels did
not produce valid results when applied to a
full-scale aircraft. The proposed hypersonic
research airplane, it was assumed, would pro-
vide a bridge over the huge technological gap
that appeared to exist between laboratory
experimentation and actual flight. 13
One aspect of the Langley proposal caused
considerable controversy. The Langley team
called for two distinct research flight profiles.
The first consisted of a variety of constant
angle-of-attack, constant altitude, and maneu-
vering flights to investigate the aerodynamic
and thermodynamic characteristics and limi-
tations of then-available technology. These
were the essential hypersonic research flights.
But the second flight profile was designed to
explore some of the problems of manned
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
The Genesis of a Research Airplane
Chapter 1
space flight by making "... long leaps out of
the sensible atmosphere." This included inves-
tigations into ". . . high-lift and low-L/D (lift
over drag; commonly called a drag coeffi-
cient) during the reentry pull-up maneuver"
which was recognized as a prime problem for
manned space flight from both a heating and
piloting perspective. 14
This brought other concerns: ". . . As the speed
increases, an increasingly large portion of the
aircraft's weight is borne by centrifugal force
until, at satellite velocity, no aerodynamic lift
is needed and the aircraft may be operated
completely out of the atmosphere. At these
speeds the pilot must be able to function for
long periods in a weightless condition, which
is of considerable concern from the aeromed-
ical standpoint." By employing a high altitude
ballistic trajectory to approximately 250,000
feet, the Langley group expected the pilot
would operate in an essentially weightless
condition for approximately two minutes.
Attitude control was another problem, since
traditional aerodynamic control surfaces
would be useless at the altitudes proposed for
the new aircraft; the dynamic pressure would
be less than 1 pound per square foot (psf). The
use of small hydrogen-peroxide thrusters for
attitude control was proposed.
While the hypersonic research aspect of the
Langley proposal enjoyed virtually unani-
mous support, it is interesting to note that the
space flight aspect was viewed in 1954 with
what can best be described as cautious toler-
ance. There were few who believed that any
space flight was imminent, and most believed
that manned space flight in particular was
many decades in the future, probably not until
the 21st century. Several researchers recom-
mended that the space flight research was pre-
mature and should be removed from the pro-
gram. Fortunately, it remained. 15
Hypersonic stability was the first problem of
really major proportion encountered in the
study. Serious instability had already been
encountered with the X-l and X-2 at Mach
numbers substantially lower than those
expected with the proposed hypersonic
research aircraft, and it was considered a
major challenge to create a solution that
would permit stable flight at Mach 7.
Researchers at Langley discovered through
3A-inoh Rad.
\ ,-.05 0.
27.4 ft
Thrust
(sea level), 1.8
Gross Weight
Fuel Weight ,
Gross Weight' 0-b
Spec. Impulse (Alc.-Lox.), 223 sec.
v max. ( B_ 5° Launch), 6800 ft/sec
Gross Weight
Fuel ■■
Wing Loading
Aspect Ratio
Thrust
feet dia.
30,000 lb
18,000 lb
48 PSF (empty)
3.0
5^,000 lb (sea level)
(3 Hermes A3A Engines)
The notional research
airplane designed by
John V. Becker's group
at Langley shows the
basis for the eventual
X-15. Note the bullet-
shaped fuselage
(similar to the X-1)
and the configuration
of the empennage.
This was the shape
most of the early wind
tunnel and analytical
studies were per-
formed against.
(NASA)
10
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 1
The Genesis of a Research Airplane
wind tunnel testing and evaluating high speed
data from earlier X-planes that an extremely
large vertical stabilizer was required if the thin
sections then in vogue for supersonic aircraft
were used. This was largely because of a rapid
loss in the lift-curve slope of thin sections as
the Mach number increased. The solution
devised by McLellan, based on theoretical
considerations of the influence of airfoil
shape on normal force characteristics, was to
replace the thin supersonic-airfoil section of
the vertical stabilizer with a 10 degree wedge
shape. Further, a variable-wedge vertical sta-
bilizer was proposed as a means of restoring
the lift-curve slope at high speeds, thus per-
mitting much smaller surfaces, which were
easier to design structurally and imposed a
smaller drag penalty on the airframe.
McLellan's calculations indicated that this
wedge shape should eliminate the disastrous
directional stability decay encountered by the
X-landX-2.
edge, very similar to the one eventually used
on the Space Shuttle orbiters. Both the brak-
ing effect and the stability derivatives could be
varied through wide ranges by variable
deflection of the wedge surfaces. The flexibil-
ity made possible by variable wedge deflec-
tion was thought to be of great value because
a primary use of the airplane would be to
study stability, control, and handling charac-
teristics through a wide range of speeds and
altitudes. 16
Two basic structural design approaches had
been debated since the initiation of the
study — first, a conventional low-temperature
design of aluminum or stainless steel protect-
ed from the high-temperature environment by
a layer of assumed insulation; and second, an
exposed hot-structure in which no attempt
would be made to provide protection, but in
which the metal used and the design approach
would permit high structural temperatures. 17
This chart was used
by Becker to demon-
strate the relative dif-
ferences between the
nominal recovery tem-
perature, compared to
the temperatures
expected to be sus-
tained by an insulated
structure and an
appropriately
designed heat-sink
skin (hot-structure).
Inconel X was the
material of choice very
early in the study.
(NASA)
Becker's group also included speed brakes as
part of the vertical stabilizers to reduce the
Mach number and heating during reentry.
Interestingly, the speed brakes originally pro-
posed by Langley consisted of a split trailing
It was found from analysis of the heating pro-
jections for various trajectories that the air-
plane would need to accommodate tempera-
tures of over 2,000 degrees Fahrenheit on the
lower surface of the wing. At the time, there
TYPICAL TEMPERATURE HISTORY
DESIGN ALTITUDE FLIGHT
3,000
LOWER SURFACE, x = I FT
-RECOVERY TEMP.
2,000-
TEMP., °F
1,000 -
HEAT- SINK SKIN
INCONEL X, t = .082"
200
TIME, SEC
300
400
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
11
The Genesis of a Research Airplane
Chapter 1
was no known insulating technique that could
meet this requirement. The Bell "double-
wall" concept where a non-load-bearing metal
sandwich acted as the basic insulator, would
later undergo extensive development, but in
1954, it was in an embryonic state and not
applicable to the critical nose and leading
edge regions. Furthermore, it required a heavy
and space-consuming supplemental liquid
cooling system. However, the study group felt
that the possibility of local failure of any insu-
lation scheme constituted a serious hazard.
Finally, the problem of accurately measuring
heat-transfer rates — one of the prime objec-
tives of the new research aircraft program —
would be substantially more difficult to
accomplish with an insulated structure.
At the start of the study it was by no means
obvious that the hot-structure approach would
prove practical either. The permissible design
temperature for the best available material was
about 1,200 degrees Fahrenheit, which was far
below the estimated equilibrium temperature
peak of about 2,000 degrees Fahrenheit. It was
clear that some form of heat dissipation would
have to be employed — either direct internal
cooling or heat absorption into the structure
itself. It was felt that either solution would
bring a heavy weight penalty.
The availability of Inconel X 18 arid its excep-
tional strength at extremely high temperatures,
made it, almost by default, the structural mate-
rial preferred by Langley for a hot-structure
design. During mid-1954, an analysis of an
Inconel X structure was begun by Becker's
group; concurrently, a detailed thermal analy-
sis was conducted. A subsequent stress study
indicated that the wing skin thickness should
range from 0.05 to 0.10 inches — about the
same values found necessary for heat absorp-
tion in the thermal analysis.
Thus it was possible to solve the structural
problem for the transient conditions of a
Mach 7 aircraft with no serious weight penal-
ty for heat absorption. This was an unexpect-
ed plus for the hot-structure. Together with the
fact that none of the perceived difficulties of
an insulated-type structure were present, the
study group decided in favor of an uninsulat-
ed hot-structure design.
Unfortunately, it later proved that the hot-
structure had problems of its own, particularly
COMPARISON OF INCONEL X WITH OTHER ALLOYS
TENSILE YIELD STRESS,
KSI
IOO
HEAT RADIATED,
BTU/SQ FT/SEC
10
TITANIUM
500
T, °F
1000
1500
Inconel X was easily
the best high-tempera-
ture alloy available
during the 1950s.lt
possessed a rare
combination of high
tensile strength and
the ability to withstand
high temperatures.
Although it proved
somewhat difficult to
work with, it did not
impose some of the
problems encountered
with titanium on other
high-speed aircraft
projects. (NASA)
12
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 1
The Genesis of a Research Airplane
in the area of nonuniform temperature distri-
bution. Detailed thermal analyses revealed
that large temperature differences would
develop between the upper and lower wing
skin during the pull-up portions of certain tra-
jectories. This unequal heating would result in
intolerable thermal stresses in a conventional
structural design. To solve this new problem,
wing shear members were devised which did
not offer any resistance to unequal expansion
of the wing skins. The wing thus was essen-
tially free to deform both spanwise and chord-
wise with asymmetrical heating. Although
this technique solved the problem of the gross
thermal stresses, localized thermal-stress
problems still existed in the vicinity of the
stringer attachments. The study indicated,
however, that proper selection of stringer pro-
portions and spacing would produce an
acceptable design free from thermal buckling.
During the Langley studies, it was discovered
that differential heating of the wing leading
edge produced changes in the natural torsion-
al frequency of the wing unless some sort of
flexible expansion joint was incorporated in
its design. The hot leading edge expanded
faster than the remaining structure, introduc-
ing a compression that destabilized the sec-
tion as a whole and reduced its torsional stiff-
ness. To negate this phenomenon, the leading
edge was segmented and flexibly mounted in
an attempt to reduce thermally induced buck-
ling and bending.
With its research objectives and structure
now essentially determined, the Langley
team turned its attention to the questions of
propulsion by examining various existing
rocket propulsion systems. The most promis-
ing configuration was found to be a grouping
of four General Electric Al or A3 Hermes
rocket engines, due primarily to the "thrust
stepping" (a crude method of modulating, or
throttling, the thrust output) option this con-
figuration provided.
The studies prompted the NACA to adopt the
official policy that the construction of a
manned hypersonic research airplane was fea-
sible. In June 1954, Dr. Hugh L. Dryden sent a
letter to Lieutenant General Donald Putt at Air
Force Headquarters stating that the NACA
was interested in the creation of a new manned
research aircraft program that would explore
hypersonic speeds and altitudes well in excess
of those presently being achieved. The letter
also recommended that a meeting between the
NACA, Air Force Headquarters, and the Air
Force SAB be arranged to discuss the project.
Putt responded favorably, and also recom-
mended that the Navy be invited to participate.
NACA representatives met with members of
the Air Force and Navy research and develop-
ment groups on 9 July 1954 to present the
proposal for a hypersonic research aircraft as
an extension of the existing cooperative
research airplane program. It was soon dis-
covered that the Air Force SAB had been
making similar proposals to Air Force
Headquarters, and that the Office of Naval
Research had already contracted with the
Douglas Aircraft Company to determine the
feasibility of constructing a manned aircraft
capable of achieving 1,000,000 feet altitude.
Douglas had concluded that 700,000 foot alti-
tudes would be possible from the reentry
deceleration standpoint, but that the thermo-
structural problem had not been thoroughly
analyzed. It was agreed that a cooperative pro-
gram would be more cost effective and likely
lead to better research data at an earlier time. 19
The Navy and Air Force representatives
viewed the NACA proposal with favor,
although each had some reservations. At the
close of the meeting, however, there was
agreement that both services would further
study the details of the NACA proposal, and
that the NACA would take the initiative to
secure project approval from the Department
of Defense. 20
Less than a month later, the Air Force identi-
fied the principal shortcoming of the original
Langley proposal — the apparent lack of a
suitable rocket engine. In early August the
Power Plant Laboratory at the Wright Air
Development Center (WADC) pointed out
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that "no current rocket engines" entirely satis-
fied the NACA requirements, and emphasized
that the Hermes engine was not designed to be
operated in close proximity to humans — that
it usually was fired only when shielded by
concrete walls. Other major objections to the
Hermes engine centered around its relatively
early state of development, its limited design
life (intended for missile use, it was not
required to operate successfully more than
once), and the apparent difficulty of incorpo-
rating the ability to throttle it during flight. 21
WADC technical personnel who visited
Langley on 9 August drew a firm distinction
between engines intended for piloted aircraft
and those designed for missiles; the NACA
immediately recognized the problem, but con-
cluded that although program costs would
increase, the initial feasibility estimates would
not be affected. 22
WADC's official reaction to the NACA pro-
posal was submitted to the Air Research and
Development Command (ARDC) on 13
August. 23 Colonel V. R. Haugen reported
"unanimous" agreement among WADC par-
ticipants that the proposal was technically fea-
sible; excepting the engine situation, there
was no occasion for adverse comment. The
evaluation forwarded by Haugen also con-
tained a cost estimate of $12,200,000 "distrib-
uted over three to four fiscal years" for two
research aircraft and necessary government-
furnished equipment. Estimated costs includ-
ed: $1,500,000 for design work; $9,500,000
for construction and development, including
flight test demonstration; $650,000 for gov-
ernment furnished equipment, including
engines, $300,000 for design studies and
specifications; and $250,000 for modification
of a carrier aircraft. 24 Somewhat prophetically,
one WADC official commented informally:
"Remember the X-3, the X-5, [and] the X-2
overran 200 percent. This project won't get
started for $12,000,000.' ,2S
On 13 September, the ARDC issued an
endorsement of the NACA proposal, and rec-
ommended that the Air Force "... initiate a
project to design, construct, and operate a new
research aircraft similar to that suggested by
NACA without delay." The aircraft, empha-
sized ARDC, should be considered a pure
research vehicle and should not be pro-
grammed as a weapon system prototype. On
4 October 1954, Brigadier General Benjamin
S. Kelsey, Deputy Director of Research and
Development at Air Force Headquarters, stat-
ed that the project would be a joint Navy-
NACA-USAF effort managed by the Air
Force and guided by a joint steering commit-
tee. Air Force Headquarters further pointed
out the necessity for funding a special flight
test range as part of the project. 26
The NACA Committee on Aeronautics met
on 5 October 1954 to consider the hypersonic
research aircraft. During the meeting, historic
and technical data were reviewed by various
committee members including Walter C.
Williams, De E. Beeler, and research pilot A.
Scott Crossfield from the High-Speed Flight
Station (HSFS). Williams' support was cru-
cial. Crossfield would later describe Williams
as "... the man of the 20th Century who made
more U.S. advanced aeronautical and space
programs succeed than all the others together.
... He had no peer. None. He was a very
strong influence in getting the X- 15 program
launched in the right direction." 27
Although one Committee member expressed
opposition to the proposed hypersonic
research aircraft as an extension to the on-
going test programs, the rest of the Committee
supported the project. The Committee formal-
ly adopted a resolution to build a Mach 7
research airplane (attached as an appendix to
this monograph). 28
Because the anticipated cost of the project
would require support from Department of
Defense contingency funds as well as Air
Force and Navy R&D funds, a formal
Memorandum of Understanding (MoU) was
drafted and sent around for signatures begin-
ning in early November 1954. The MoU was
originated by Trevor Gardner (Air Force
Special Assistant for Research and
Development), and was forwarded, respec-
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Chapter 1
The Genesis of a Research Airplane
tively, for the signatures of J. H. Smith Jr. 29
(Assistant Secretary of the Navy [Air]) and
Hugh L. Dryden (Director of the NACA).
Dryden signed the MoU on 23 December
1954, and returned executed copies to the Air
Force and Navy. 30
The MoU (attached as an appendix to this
monograph) provided that technical direction
of the research project would be the responsi-
bility of the NACA, acting "... with the advice
and assistance of a Research Airplane
Committee" composed of one representative
each from the Air Force, Navy, and the NACA.
Administration of the design and construction
phases of the project was assigned to the Air
Force. The NACA would conduct the flight
research, with extensive support from the Air
Force Flight Test Center. The Navy was essen-
tially left paying 25 percent of the bills with
littie active roll in the project, although it
would later supply biomedical expertise and a
single pilot. The NACA and the Research
Airplane Committee were charged with the
responsibility for disseminating the research
results to the military services and aircraft
industry as appropriate based on various secu-
rity aspects. The concluding statement on the
MoU was: "Accomplishment of this project is
a matter of national urgency." 31
It should be noted that it was not unusual in
the late 1940s and early 1950s for the military
services to fund the development and con-
struction of aircraft for the NACA to use in its
flight test programs. This was how most of the
testing on the X-l and others had been accom-
plished. The eventual X-l 5 would be the
fastest, highest-flying, and most expensive of
these joint projects. 32
After the signed copies of the MoU were
returned to all participants, the Department
of Defense authorized the Air Force to issue
invitations to contractors having experience
in the development of fighter-type aircraft to
participate in the design competition. After
the Christmas holidays, on 30 December, the
Air Force sent invitation-to-bid letters to
12 prospective contractors; Bell, Boeing,
Chance-Vought, Consolidated (Convair),
Douglas, Grumman, Lockheed, Martin,
McDonnell, North American, Northrop, and
Republic. The letter asked those interested in
bidding to notify Wright Field by 10 January
1955, and to attend a bidder's conference on
18 January 1955. 33
Attached to the letter were a preliminary out-
line specification, an abstract of the Langley
preliminary study, a discussion of possible
engines, a list of data requirements, and a cost
outline statement. Each bidder was required to
satisfy various requirements set forth, except
in the case of the NACA abstract which was
presented as "... representative of possible
solutions." 34
Grumman, Lockheed, and Martin expressed
little interest in the competition and did not
attend the bidder's conference, leaving nine
possible competitors. At the bidders' confer-
ence, representatives from the contractors
met with NACA and Air Force personnel to
discuss the competition and the basic design
requirements.
During the bidders' conference, the airframe
manufacturers were informed that one prime
proposal and one alternate proposal (that
might offer an unconventional but superior
solution to the problems involved) would be
accepted from each company. It also was
noted that an engineering study, only, would
be required for a modified aircraft where an
observer could be substituted for the
research instrumentation (a Navy require-
ment); that a weight allowance of 800
pounds, a volume of 40 cubic feet, and a
power requirement of 2.25 kilowatts (kW)
needed to be provided for research instru-
mentation; and that the winning design
would have to be built in 30 months and be
capable of attaining speeds of Mach 6 and
altitudes of 250,000 feet. Following the pre-
liminary statements concerning the bidding,
NACA personnel briefed the various compa-
nies in attendance on new information that
had resulted from late 1954 wind tunnel
research that had taken place at Langley.
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Subsequently, between the bidders' confer-
ence and the 9 May submission deadline,
Boeing, Chance-Vought, Convair, Grumman,
McDonnell, and Northrop notified the Air
Force that they did not intend to submit for-
mal proposals. This left Bell, Douglas, North
American, and Republic. During this period,
representatives from these companies met
with NACA personnel on numerous occa-
sions and reviewed technical information on
various aspects of the forthcoming research
airplane. The NACA also provided these con-
tractors with further information gained as a
result of wind tunnel tests in the Ames 10-by-
14 inch supersonic tunnel and the Langley
Mach 4 blowdown tunnel.
On 17 January 1955, NACA representatives
met with Air Force personnel at Wright Field
and were informed that the research airplane
was identified as Air Force Project 1226 and
would be officially designated X-15.
The Power Plant Laboratory had originally
listed the Aerojet XLR73, Bell XLR81, North
American NA-5400 (an engine in early devel-
opment, still lacking a military designation),
and the Reaction Motors XLR10 (and its vari-
ants, including the XLR30) as engines that the
airframe competitors could use in their
designs. Early in January, the laboratory had
become concerned that the builders of engines
other than those listed might protest the exclu-
sion of their products. Consequently there
emerged an explanation and justification of the
engine selection process. It appeared that the
engineers had confidence in the ability of the
XLR81 and XLR73 to meet airplane require-
ments, had doubts about the suitability of the
XLR25 (a Curtiss-Wright product), and held
the thrust potential of the XLR8 and XLR11
(similar engines) in low repute. For practical
purposes, this exhausted the available Air
Force-developed engines suitable for manned
aircraft. The XLR10 and NA-5400 were the
only Navy-developed engines viewed as
acceptable in terms of the competition. 35
engines originally listed as suitable for the
X-15 program, 36 and this information was dis-
tributed to all four prospective airframe con-
tractors. 37 Due to its early development status,
there was little data available for the North
American NA-5400, and the Reaction Motors
XLR10 was "not recommended" at the sug-
gestion of the engine manufacturer itself. On
4 February each of the prospective engine
contractors (Aerojet, Bell, North American,
and Reaction Motors) was asked to submit an
engine development proposal. 38 Based on this,
the Air Force very slightly relaxed the rigid
limitations on engine selection, instructing
competitors that "... if ... an engine not on the
approved list offers sufficient advantage, the
airframe company may, together with the
engine manufacturer, present justification for
approval . . ." to the Air Force. 39
On 9 May 1955, Bell, Douglas, North
American, and Republic submitted their pro-
posals to the Air Force. Two days later the
technical data was distributed to the evaluation
groups with a request that results be returned
by 22 June. 40 The final evaluation meeting was
scheduled for 25 July at Wright Field. 41
Shortly thereafter, Hartley A. Soule, as
Chairman of the NACA evaluation group,
sent the evaluation rules and processes to the
NACA laboratories. The evaluation would be
based on the technical and manufacturing
competency of each contractor, schedule and
cost estimates, design approach, and the
research utility of each design. In order to
expedite the evaluation, each of the NACA
laboratories was assigned specific items to
consider with responses to be returned to
Soule no later than 13 June.
The evaluation of the engine would be made
at the same time, but would be conducted sep-
arate from that of the airframe contractor, with
the possibility that the chosen engine might
not be the one selected by the winning air-
frame contractor.
Earlier, the engine manufacturers had been
contacted for specific information about the
On 10 June the HSFS results were sent to
Soule, based on the design approach and
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Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter I
The Genesis of a Research Airplane
research utility aspects of the airframe, flight
control system, propulsion unit, crew provi-
sions, handling and launching, and miscella-
neous systems. The proposals were ranked:
(1) Douglas; (2) North American; (3) Bell;
and (4) Republic. The proposals from
Douglas and North American were consid-
ered almost equal on the basis of points.
The Ames final evaluation, on 13 June 1955,
ranked the proposals: (1) North American;
(2) Douglas; (3) BeU; and (4) Republic. The
North American structure was considered to
be more representative of future aircraft and
thus superior in terms of research utility.
Douglas retained a simple and conventional
magnesium structure, but in so doing avoided
the very thermodynamic problems the
research effort wished to explore.
The 14 June final evaluation from Langley
ranked the proposals: (1) North American; (2)
Douglas; (3) Republic; and (4) Bell. Langley
felt that while the magnesium wing structure
of Douglas was feasible, it was feared that
local hot spots caused by irregular aerody-
namic heating could weaken the structure and
be subject to failure. North American's use of
Inconel X was believed to be an advantage.
The final order representing the overall
NACA evaluation was (1) North American;
(2) Douglas; (3) Bell; and (4) Republic. All
of the laboratories involved in this portion of
the evaluation considered both the North
American and Douglas proposals to be
much superior to those submitted by Bell
and Republic.
As with the NACA evaluations, the Air Force
found little difference between the Douglas
and North American designs, point-wise, with
both proposals significantly superior to those
of Bell and Republic. The Navy evaluation
found much the same thing, ranking the pro-
posals: (1) Douglas; (2) North American; (3)
Republic; and (4) Bell.
On 26-28 July, the Air Force, Navy, and
NACA evaluation teams met to coordinate
their separate results. The Air Force and the
NACA concluded that the North American
proposal best accommodated their require-
ments. Accordingly, the Navy decided not to
be put in the position of casting the dissenting
vote and after short deliberation, agreed to go
along with the decision of the Air Force and
the NACA. A combined meeting of the Air
Force, Navy, and the NACA was held at
NACA Headquarters on 12 August for the
final briefing on the evaluation. Later, the
Research Airplane Committee met, accepted
the findings of the evaluation groups, and
agreed to present the recommendation to the
Department of Defense.
Interestingly, the North American proposal
was by far the most expensive. The estimat-
ed costs for three aircraft plus one static test
article and supporting equipment were: Bell,
$36.3 million; Douglas, $36.4 million;
Republic, $47 million; and North American,
$56.1 rnillion.
Because the estimated costs submitted by
North American were far above the amount
allocated for the project, the Research
Airplane Committee included a recommenda-
tion for a funding increase that would need to
be approved before the actual contract was
signed. A further recommendation, one that
would later take on greater importance, called
for relaxing the proposed schedule by up to
one-and-one-half years. These recommenda-
tions were sent to the Assistant Secretary of
Defense for Research and Development.
Events took an unexpected twist on 23
August when the North American represen-
tative in Dayton verbally informed the Air
Force that the company wished to withdraw
its proposal. On 30 August, North American
sent a letter to the Air Force formally
requesting that the company be allowed to
withdraw from consideration. 42
The Vice President and Chief Engineer for
North American, Raymond H. Rice, wrote to
the Air Force on 23 September and explained
that the company had decided to withdraw
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The Genesis of a Research Airplane
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from the competition because it had recently
won new bomber and long range interceptor
competitions and also had increased activity
relating to its on-going F-107 fighter. Having
undertaken these projects, North American
said it would be unable to accommodate the
fast engineering man-hours build-up that
would be required to support the desired
schedule. Rice went on that, "... due to the
apparent interest that has subsequently been
expressed in the North American design, the
contractor [North American] wishes to extend
two alternate courses which have been previ-
ously discussed with Air Force personnel: The
engineering man-power work load schedule
has been reviewed and the contractor wishes
to point out that Project 1226 could be han-
dled if it were permissible to extend the
schedule... over an additional eight month
period; in the event the above time extension
is not acceptable and in the best interest of the
project, the contractor is willing to release the
proposal data to the Air Force at no cost." 43
As it turned out, the possibility of extending
the schedule had already been approved on
12 August, allowing North American to with-
draw its previous letter of retraction once it
had been officially informed that it had won
the contract. 44 Accordingly, on 30 September
1955, the Air Force formally notified North
American that its design had been selected as
the winner. The other bidders were conse-
quently notified of North American's selec-
tion and thanked for their participation. 45
By 11 October, the estimate from North
American had been reduced from
$56,000,000 to $45,000,000 and the maxi-
mum annual funds requirement from
$26,000,000 to $15,000,000. Shortly there-
after, the Department of Defense released the
funds needed to start work. More meetings
between the Air Force, the NACA, and North
American were held on 27-28 October, large-
ly to define changes to the aircraft configura-
tion. On 18 November, letter contract
AF33(600)-31693 was sent to North
American, and an executed copy was returned
on 8 December 1955. 46 The detailed design
and development of the hypersonic research
airplane had been underway for just under a
year at this point. 47
On 1 December 1955, a series of actions 48
began that resulted in letter contract
AF33(600)-32248 being sent to Reaction
Motors, effective on 14 February 1956. Its ini-
tial allocation of funds totaled $3,000,000,
with an eventual expenditure of about
$6,000,000 foreseen as necessary for the
delivery of the first flight engine. 49
A definitive contract for North American was
completed on 11 June 1956, superseding the
letter contract and two intervening amend-
ments. At that time, $5,315,000 had been
committed to North American. The definitive
contract allowed the eventual expenditure of
$40,263,709 plus a fee of $2,617,075. For this
sum, the government was to receive three
X-15 research aircraft, a high speed and a low
speed wind tunnel model program, a free-spin
model, a full-size mockup, propulsion system
tests and stands, flight tests, modification of a
B-36 carrier aircraft, a flight handbook, a
maintenance handbook, technical data, peri-
odic reports of several types, ground handling
dollies, spare parts, and ground support equip-
ment. Exclusive of contract costs were fuel
and oil, special test site facilities, and expens-
es to operate the B-36. The delivery date for
the X-15s was to be 31 October 1958. The
quantity of aircraft had been determined by
experience; it had been noted during earlier
research aircraft programs that two aircraft
were enough to handle the anticipated work-
load, but three assured that the test pace could
be maintained even with one aircraft down. 50
This lesson has been largely forgotten in our
current budget-conscious era.
A final contract for the engine, the prime unit
of government furnished equipment, was
effective on 7 September 1956. Superseding
the letter contract of February, it covered the
expenditure of $10,160,030 plus a fee of
$614,000. 51 For this sum, Reaction Motors
agreed to deliver one engine, a mockup,
reports, drawings, and tools.
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Chapter 1
Notes and
References
On 3 March 1915, Congress passed a Public law establishing "an Advisory Committee for Aeronautics." As stipu-
lated in the Act, the purpose of this committee was "... to supervise and direct the scientific study of the problems
of flight with a view to their practical solution."
John V. Becker, "The X-15 Program in Retrospect" (paper presented at the 3rd Eugen Sanger Memorial Lecture,
Bonn, Germany, 4-5 December 1968), p. 1.
This ignores the research by the Air Force and Bell Aircraft into such skip-glide concepts as Bomber-Missile
(BoMi) and Rocket-Bomber (RoBo), both of which were highly classified, and ultimately would be folded into the
Dyna-Soar program.
The accepted standard at the time was to report altitudes in statute miles.
This had been the Langley Memorial Aeronautical Laboratory until July 1948 when the "Memorial" was dropped.
John V. Becker, "The X-15 Program in Retrospect" (a paper presented at the 3rd Eugen Sanger Memorial Lecture,
Bonn, Germany, 4-5 December 1968), p. 2.
Letter from John V. Becker to Dennis R. Jenkins, 12 June 1999.
Preliminary Outline Specification for High-Altitude, High-Speed Research Airplane, NACA Langley, 15 October
1954.
General Requirements for a New Research Airplane, NACA Langley, 11 October 1954.
John V. Becker, "The X-15 Program in Retrospect" (paper presented at the 3rd Eugen Sanger Memorial Lecture,
Bonn, Germany, 4-5 December 1968), p. 2.
This was well below the B-36's maximum useful load of 84,000 pounds, especially when it is considered that all
military equipment on the B-36 would have been removed as well. But the conservative estimate allowed the car-
rier aircraft to fly high and fast enough to provide a decent launch platform for the research airplane.
Dr. Hugh L. Dryden, "General Background of the X-15 Research Airplane Project" (a paper presented at the NACA
Conference on the Progress of the X-15 Project, Langley Aeronautical Laboratory, Langley Field, Virginia, 25-26
October 1956), pp. xvii-xix and 3-9.
John V. Becker, "Review of the Technology Relating to the X-15 Project" (a paper presented at the NACA
Conference on the Progress of the X-15 Project, Langley Aeronautical Laboratory, Langley Field, Virginia, 25-26
October 1956, p. 3; John V. Becker, "The X-15 Program in Retrospect" (paper presented at the 3rd Eugen Sanger
Memorial Lecture, Bonn, Germany, 4-5 December 1968), p. 7.
John V. Becker, "The X-15 Program in Retrospect" (paper presented at the 3rd Eugen Sanger Memorial Lecture,
Bonn, Germany, 4-5 December 1968), p. 2.
Dr. Hugh L. Dryden, "Toward the New Horizons of Tomorrow," First von Karman Lecture, Astronautics . January
1963.
John V. Becker, "Review of the Technology Relating to the X-15 Project" (a paper presented at the NACA
Conference on the Progress of the X-15 Project, Langley Aeronautical Laboratory, Langley Field, Virginia, 25-26
October 1956, pp. 4-5.
These" same trade studies would be repeated many times during the concept definition for Space Shuttle.
Inconel X s is a nickel-chromium alloy whose name is a registered trademark of Huntington Alloy Products
Division, International Nickel Company, Huntington, West Virginia.
Dr. Hugh L. Dryden, "General Background of the X-15 Research Airplane Project" (a paper presented at the NACA
Conference on the Progress of the X-15 Project, Langley Aeronautical Laboratory, Langley Field, Virginia, 25-26
October 1956), pp. xvii-xix.
1 Memorandum from J. W. Rogers, Liquid Propellant and Rocket Branch, Rocket Propulsion Division, Air Research
and Development Command (ARDC), to Lieutenant Colonel L. B. Zambon, Power Plant Laboratory, Wright Air
Development Center (WADC), 13 July 1954, in the files of the AFMC History Office, Wright-Patterson AFB, Ohio.
Letter from Colonel P. F. Nay, Acting Chief, Aeronautics and Propulsion Division, Deputy Commander of
Technical Operations, ARDC, to Commander, WADC, 29 July 1954, subject: New Research Aircraft, in the files
of the AFMC History Office, Wright-Patterson AFB, Ohio.
'■ Memorandum from J. W. Rogers, Liquid Propellant and Rocket Branch, Rocket Propulsion Division, Power Plant
Laboratory, to Chief, Non-Rotating Engine Branch, Power Plant Laboratory, WADC, 11 August 1954, subject:
Conferences on 9 and 10 August 1954 on NACA Research Aircraft-Propulsion System, in the files of the AFMC
History Office, Wright-Patterson AFB, Ohio.
1 A published summary of the 9 July NACA presentations did not appear until 14 August.
1 Letter from Colonel V. R. Haugen to Commander, ARDC, 13 August 1954, in the files of the AFMC History Office,
Wright-Patterson AFB, Ohio.
' Memorandum from R. L. Schulz, Technical Director of Aircraft, to Chief, Fighter Aircraft Division, Director of
WSO, WADC, not dated (presumed about 13 August 1954), in the files of the AFMC History Office, Wright-
Patterson AFB, Ohio.
s Letter from Brigadier General Benjamin S. Kelsey, Deputy Director of R&D, DCS/D, USAF, to Commander,
ARDC, 4 October 1954, subject: New Research Aircraft, in the files of the AFMC History Office, Wright-Patterson
AFB, Ohio.
' Letter from A. Scott Crossfield to Dennis R. Jenkins, 30 June 1999.
! Dr. Hugh L. Dryden, "General Background of the X-15 Research Airplane Project" (a paper presented at the NACA
Conference on the Progress of the X-15 Project, Langley Aeronautical Laboratory, Langley Field, Virginia, 25-26
October 1956), pp. xvii-xix.
' It was common practice in the 1950s to only record the last name and initials for individuals on official corre-
spondence. Where possible first names are provided; but in many cases a first name cannot be definitively deter-
mined from available documentation.
Memorandum from Trevor Gardner, Special Assistant for R&D, USAF, to J. H. Smith Jr., Assistant Secretary Navy
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
19
The Genesis of a Research Airplane Chapter I
(Air), 9 November 1954, subject: Principles for the Conduct of a Joint Project for a New High Speed Research
Airplane; Letter from J. H. Smith Jr., Assistant Secretary Navy (Air), to Dr. Hugh L. Dryden, Director of NACA,
21 December 1954; Letter from Dr. Hugh L. Dryden to Trevor Gardner returning a signed copy of the MoU, 23
December 1954, in the files of the NASA History Office, Washington, DC.
31 Memorandum of Understanding, signed by Hugh L. Dryden, Director of NACA, J. H. Smith Jr., Assistant Secretary
Navy (Air), and Trevor Gardner, Special Assistant for R&D, USAF, 23 December 1954, subject: Principles for the
Conduct by the NACA, Navy, and Air Force of a Joint Project for a New High-Speed Research Airplane, in the files
of the NASA History Office, Washington, DC.
32 Walter C. Williams, "X-15 Concept Evolution" (a paper in the Proceedings of the X-15 30th Anniversary
Celebration, Dryden Flight Research Facility, Edwards, California, 8 June 1989, NASA CP-3105), p. 11.
33 Memorandum from A. L. Sea, Assistant Chief, Fighter Aircraft Division, to Director of Weapons Systems Office,
WADC, 29 December 1954, subject: New Research Aircraft, in the files of the AFMC History Office, Wright-
Patterson AFB, Ohio.
34 Letter from Colonel C. F. Damberg, Chief, Aircraft Division, Air Materiel Command, to Bell Aircraft Corporation,
et al., 30 December 1954, subject: Competition for New Research Aircraft; Memorandum from A. L. Sea, to
Director of WSO, WADC, 29 December 1954; Letter from J. B. Trenholm, Chief, New Development Office,
Fighter Aircraft Division, Director of WSO, WADC, to Commander, ARDC, 13 January 1955, subject: New
Research Aircraft, in the files of the AFMC History Office, Wright-Patterson AFB, Ohio.
35 Memorandum from J. W Rogers to Chief, Power Plant Laboratory, 4 January 1955, in the files of the AFMC
History Office, Wright-Patterson AFB, Ohio.
36 Letter from W. P. Turner, Manager, Customer Relations and Contracts Division, Reaction Motors, Inc. (hereafter
cited as RMI), to Commander, AMC, 3 February 1955, subject: XLR30 Rocket Engine — Information Concerning,
in the files of the AFMC History Office, Wright-Patterson AFB, Ohio.
37 Letter from J. B. Trenholm, Chief, New Development Office, Fighter Aircraft Division, Director of WSO, WADC,
to Bell, et al., 22 March 1955, subject: Transmittal of Data, Project 1226 Competition, in the files of the AFMC
History Office, Wright-Patterson AFB, Ohio.
38 Letter from R. W. Walker, Chief, Power Plant Development Section, Power Plant Branch, Aero-Equipment
Division, AMC, to RMI, 4 February 1955, subject: Power Plant for New Research Airplane, in the files of the
AFMC History Office, Wright-Patterson AFB, Ohio.
35 Letter from Colonel C. F. Damberg, Chief, Aircraft Division, AMC, to Bell, et al., 2 February 1955, subject: Project
1226 Competition, in the files of the'AFMC History Office, Wright-Patterson AFB, Ohio.
" Letter from Hugh L. Dryden, Director of NACA, to Deputy Director of R&D, DCS/D, USAF, 20 May 1955, no
subject; Letter from Rear Adm. R. S. Hatcher, Assistant Chief of R&D, BuAer, USN, to Commander, WADC, 31
May 1955, subject: Agreements Reached by "Research Airplane Committee," on Evaluation Procedure for X-15
Research Airplane Proposals, in the files of the AFMC History Office, Wright-Patterson AFB, Ohio.
41 Memorandum from Brigadier General Howell M. Estes Jr., Director of WSO, to Director of Laboratories, WADC
28 June 1955, subject: X-15 Evaluation, in the files of the AFMC History Office, Wright-Patterson AFB, Ohio.
42 Memorandum from Colonel C. G. Allen, Chief, Fighter Aircraft Division, Director of Systems Management,
ARDC, to Commander, WADC 23 August 1955, subject: X-15 Evaluation, in the files of the AFMC History Office,
Wright-Patterson AFB, Ohio.
43 Letter from R. H. Rice, Vice President and Chief Engineer, North American Aviation, Inc., to Commander, ARDC,
23 September 1955, subject: Project 1226, Research Airplane, in the files of the AFMC History Office, Wright-
Patterson AFB, Ohio.
44 X-15 WSPO Weekly Activity Report, 22 September 1955, in the files of the AFMC History Office, Wright-
Patterson AFB, Ohio.
45 Letter from Colonel C. F. Damberg, Chief, Aircraft Division, AMC, to North American Aviation, 30 September
1955, subject: X-15 Competition, in the files of the AFMC History Office, Wright-Patterson AFB, Ohio.
46 X-15 WSPO Weekly Activity Report, 13 October 1955; 20 October 1955; 27 October 1955; and 15 December
1955; Letter from N. Shropshire, Director of Contract Administration, NAA, to Commander, AMC, 8 December
1955, subject: Letter Contract AF33(600)-31693, in the files of the AFMC History Office, Wright-Patterson .AFB,
Ohio.
"' Dr. Hugh L. Dryden, "General Background of the X-15 Research Airplane Project" (a paper presented at the NACA
Conference on the Progress of the X-15 Project, Langley Aeronautical Laboratory, Langley Field, Virginia, 25-26
October 1956), pp. xviii.
48 Memorandum from Captain Chester E. McCoEough Jr., Project Officer, New Development Office, Fighter Aircraft
Division, Director of Systems Management, ARDC, to Chief, Non-Rotating Engine Branch, Power Plant
Laboratory, Director of Laboratories, WADC, 1 December 1955, subject: Engine for X-15, in the files of the AFMC
History Office, Wright-Patterson AFB, Ohio.
49 Contract AF33(600)-32248, 14 February 1956, in the files of the AFMC History Office, Wright-Patterson AFB,
Ohio.
50 Contract AF33(600)-3 1693, 1 1 June 1956, in the files of the AFMC History Office, Wright-Patterson AFB, Ohio.
51 The final fee paid to Reaction Motors was greater than the original estimate for the total engine development pro-
gram; the definitive contract exceeded more than 20 times the original estimate, and more than twice the original
program approval estimate. As events later demonstrated, even this erred badly on the side of underestimation.
20 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 2
X'15 Design and Development
Chapter 2
X-15 Design and Development
By the time of the first
industry conference in
1 956, this was the
design baseline for
the North American
X-15. Note the tall ver-
tical stabilizer, and the
fact that it does not
have the distinctive
wedge shape of the
final unit. Also notice
how far forward the
fuselage tunnels
extend — well past the
canopy. (NASA)
Harrison A. "Stormy" Storms, Jr. led the
North American X-15 design team, along with
project engineer Charles H. Feltz. These two
had a difficult job ahead of them, for although
giving the appearance of having a rather sim-
ple configuration, the X-15 was perhaps the
most technologically complex single-seat air-
craft of its day. Directly assisting Storms and
Feltz was test pilot A. Scott Crossfield, who
had worked for the NACA prior to joining
North American with the intended purpose of
working on the X-15 program. Crossfield
describes Storms as "... a man of wonderful
imagination, technical depth, and courage . . .
with a love affair with the X-15. He was a
tremendous ally and kept the objectivity of the
program intact ...." According to Crossfield,
Feltz was "... a remarkable 'can do and did'
engineer who was very much a source of the
X-15 success story." 1
Storms himself remembers his first verbal
instructions from Hartley Soule: "You have a
little airplane and a big engine with a large
thrust margin. We want to go to 250,000 feet
altitude and Mach 6. We want to study aero-
dynamic heating. We do not want to worry
about aerodynamic stability and control, or
the airplane breaking up. So if you make any
errors, make them on the strong side. You
should have enough thrust to do the job."
Adds Storms, "and so we did." 2
Crossfield's X-15 input proved particularly
noteworthy during the early days of the
development program as his experience per-
mitted the generation of logical arguments
that led to major improvements to the X-15.
He played a key role, for instance, in con-
vincing the Air Force that an encapsulated
ejection system was both impractical and
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
21
X'15 Design and Development
Chapter 2
unnecessary. His arguments in favor of an
ejection seat, capable of permitting safe
emergency egress at speeds between 80 mph
and Mach 4 and altitudes from sea level
to 120,000 feet, saved significant money,
weight, and development time.
There has been considerable interest over
whether Crossfield made the right decision
in leaving the NACA since it effectively
locked him out of the high-speed, high-alti-
tude portion of the X-15 flight program.
Crossfield has no regrets: "... I made the
right decision to go to North American. I am
an engineer, aerodynamicist, and designer by
training ... While I would very much have
liked to participate in the flight research pro-
gram, I am pretty well convinced that I was
needed to supply a lot of the impetus that
allowed the program to succeed in timeli-
ness, in resources, and in technical return. . . .
I was on the program for nine years from
conception to closing the circle in flight test.
Every step: concept, criteria, requirements,
performance specifications, detailed specifi-
cations, manufacturing, quality control, and
flight operations had all become an [obses-
sion] to fight for, protect, and share — almost
with a passion." 3
Although the first, and perhaps the most
influential pilot to contribute to the X-15
program, Crossfield was not the only one to
do so. In fact, all of the initially assigned
X-15 pilots participated in the development
phases, being called on to evaluate various
operational systems and approaches, as well
as such factors as cockpit layout, control sys-
tems, and guidance schemes. They worked
jointly with engineers in conducting the sim-
ulator programs designed to study the
aspects of planned flight missions believed
to present potential difficulties. A fixed-
base simulator was developed at North
American's Los Angeles facility, containing
a working X-15 cockpit and control system
that included actual hydraulic and control-
system hardware. Following use at North
American, it was subsequently relocated to
the Flight Research Center 4 (FRC) at
Edwards AFB. Once flight research began,
the simulator was constantly refined with the
results of the flight test program, and late in
its life the original analog computers were
replaced by much faster digital units. For the
life of the program, every X-15 flight was
preceded by 10-20 hours in the simulator.
A ground simulation of the dynamic envi-
ISVS-447LI
49 FT
THREE VIEW
PERFORMANCE
VELOCITY
DESIGN ALTITUDE
LANDING SPEED
POWER PLANT-RM1
MAX THRUST (40,000 FT)
MIN THRUST (40,000 FT)
WING
AREA
SWEEP e/4.
THICKNESS
ASPECT RATIO
WEIGHT
LAUNCHING
BURN-OUT
PR0PELLANT
6600 FT PER SEC
250,000 FT
164 KN
57.000 LB
17,000 LB
200 $Q FT
25 DEGREES
5 PERCENT
25
31.275 LB
12.971 LB
18.304 LB
One of the more con-
troversial features of
the North American
design was the fuse-
lage tunnels that car-
ried the propellant
lines and engine con-
trols around the full
monocoque propellant
tanks, shown in this
1956 sketch. Originally
these tunnels extend-
ed forward ahead of
the cockpit, and the
NACA worried they
would create unac-
ceptable vortices.
(NASA)
22
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 2
X'15 Design and Development
The interior layout of
the fuselage did not
change much after the
1956 conference. Note
the helium tank locat-
ed in the middle of the
LOX tank. The hydro-
gen-peroxide (H202)
was used to power the
turbopump on the
XLR99 rocket engine.
(NASA)
ronment was provided by use of the Navy
centrifuge at the Naval Air Development
Center (NADC) Johnsville, Pennsylvania.
Over 400 simulated reentries 5 were flown
during an initial round of tests completed on
12 July 1958; Iven Kincheloe, Joe Walker,
Scott Crossfield, Al White, Robert White,
Neil Armstrong, and Jack McKay participat-
ed. The primary objective of the program
was to assess the pilot's ability to make
emergency reentries under high dynamic
conditions following a failure of the stability
augmentation system. The results were gen-
erally encouraging. 6
When the contracts with North American
had been signed, the X-15 was some three
years away from actual flight test. Although
most of the basic research into materials and
structural science had been completed, a
great deal of work remained to be accom-
plished. This included the development of
fabrication and assembly techniques for
Inconel X and the new hot-structure design.
North American met the challenge of each
problem with a practical solution, and even-
tually some 2,000,000 engineering man-
hours and 4,000 wind tunnel hours in 13 dif-
ferent wind tunnels were logged.
The original North American proposal gave
rise to several questions which prompted a
meeting at Wright-Patterson AFB on 24-25
October 1955. Subsequent meetings were
held at the North American Inglewood plant
on 28-29 October and 14-15 November.
Major discussion items included North
American's use of fuselage tunnels and all-
moving horizontal stabilizers (the "rolling-
tail"). The rolling-tail operated differentially
to provide roll control, and symmetrically to
provide pitch control; this allowed the elimi-
nation of conventional ailerons. North
American had gained considerable experi-
ence with all-moving control surfaces on the
YF-107A fighter. In this instance the use of
differentially operated surfaces simplified
the construction of the wing, and allowed
elimination of protuberances that would
have been necessary if aileron actuators had
been incorporated in the thin wing. Such pro-
tuberances would have disturbed the airflow
and created another heating problem.
One other significant difference between the
configuration of the NACA design and that
of the actual X-15 stemmed from North
American's use of full-monocoque propel-
lant tanks in the center fuselage and the use
SYS-4471 1
INBOARD PROFILE
LOX
t^.#!HU
■SPACE
CONTROL
ROCKETS
-EQUIPMENT
BAY
H 2 r
R0CKET ENGINE-
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
23
X-15 Design and Development
Chapter 2
of tunnels on both sides of the fuselage to
accommodate the propellant lines and engine
controls that ordinarily would have been
contained within the fuselage. The NACA
expressed concern that the tunnels might cre-
ate undesirable vortices that would interfere
with the vertical stabilizer, and suggested
that the tunnels be kept as short as possible
in the area ahead of the wing. North
American agreed to make the investigation
of the tunnels' effects a subject of an early
wind tunnel-model testing program. 7
During the spring and summer of 1956, sev-
eral scale models were exposed to rather
intensive wind tunnel tests. A 1/50-scale-
model was tested in the 11 -inch hypersonic
and 9-inch blowdown tunnels at Langley,
and another in a North American wind tun-
nel. A 1/15-scale model was also tested at
Langley and a rotary-derivative model was
tested at Ames. The various wind tunnel pro-
grams included investigations of the speed
brakes, horizontal stabilizers without dihe-
dral, several possible locations for the hori-
zontal stabilizer, modifications of the vertical
stabilizer, the fuselage tunnels, and control
effectiveness, particularly of the rolling-tail.
Another subject in which there was consid-
erable interest was determining the cross-
section radii for the leading edges of the var-
ious surfaces.
On 1 1 June 1956, North American received a
production go-ahead for the three X-15 air-
frames (although the first metal was not cut
for the first aircraft until September). Four
days later, on 15 June 1956, the Air Force
assigned three serial numbers (56-6670
through 56-6672) to the X-15 program. 8
By July, the NACA felt that sufficient
progress had been made on the X-15 devel-
opment to make an industry conference on
the project worthwhile. 9 The first Conference
on the Progress of the X-15 Project was held
WIND-TUNNEL PROGRAM
NAA8.75XII-FT
LAL 8-FT TRANS.
NAA SAL 16-IN.
MIT NSL
LAL HIGH M JET
AAL I0XI4-IN.
LALH-IN.HYR
CONFIGURATIONS
I
WEZ 2
hmw/MMmM
)>»/»»»A
Jsh
CONFIGURATIONS
I
2
SPEED BRAKES OPEN
8b s 45°
Seven different wind
tunnels are represent-
ed in this chart show-
ing how the extreme
front of the fuselage
tunnels began to be
modified. Note the
large speed brakes on
the vertical stabilizer.
"LAL' on the chart is
the Langley
Aeronautical
Laboratory, while
"AAL' is the Ames
Aeronautical
Laboratory.
(NASA)
24
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 2
X'15 Design and Development
at Langley on 25-26 October 1956. There
were 313 attendees representing the Air
Force, the NACA, Navy, various universities
and colleges, and most of the major aero-
space contractors. It was evident from the
papers that a considerable amount of
progress had already been made, but that a
few significant problems still lay ahead. 10
A comparison of the suggested configuration
contained in the original NACA proposal
and the North American configuration pre-
sented to the industry conference revealed
that the span of the X-15 had been reduced
from 27.4 feet to only 22 feet and that the
North American fuselage had grown from
the suggested 47.5-foot overall-length to
49 feet. North American followed the NACA
suggestion by selecting Inconel X as the
major structural material and in the design of
a multispar wing with extensive use of cor-
rugated webs."
One of the papers summarized the aerody-
namic characteristics that had been obtained
by tests in eight different wind tunnels. 12
These tests had been made at Mach numbers
ranging from less than 0.1 to about 6.9, and
investigated such problems as the effects of
speed brake deflection on drag, the lift-drag
relationship of the entire aircraft, of individ-
ual components such as the wings and fuse-
lage tunnels, and of combinations of individ-
ual components. One of the interesting prod-
ucts was a finding that almost half of the
total lift at high Mach numbers would be
derived from the fuselage tunnels. Another
result was the confirmation of the NACA's
prediction that the original fuselage tunnels
would cause longitudinal instability; for sub-
sequent testing the tunnels had been short-
ened in the area ahead of the wing, greatly
reducing the instability. Still other wind tun-
nel tests had been conducted in an effort to
establish the effect of the vertical and hori-
zontal tail surfaces on longitudinal, direc-
tional, and lateral stability.
It should be noted that wind tunnel testing in
the late 1950s was, and still is, an inexact sci-
ence. For example, small (3- to 4-inch) mod-
els of the X-15 were "flown" in the hyper-
velocity free-flight facility at Ames. The
models were made out of cast aluminum,
cast bronze, or various plastics, and were
actually fairly fragile. Despite this, the goal
was to shoot the model out of a gun at
tremendous speeds in order to observe shock
wave patterns across the shape. As often as
not, what researchers saw were pieces of
X-15 models flying down the range side-
ways. Fortunately, enough of the models
remained intact to acquire meaningful data. 13
Other papers presented at the industry con-
ference dealt with research into the effect of
the aircraft's aerodynamic characteristics on
the pilot's control. Pilot-controlled simula-
tion flights for the exit and reentry phases
had been conducted; researchers reported
that the pilots had found the early configura-
tions nearly uncontrollable without damping,
and that even with dampers the airplane pos-
sessed only minimum stability during parts
These charts show
the expected tempera-
tures and skin thick-
ness for various parts
of the X-15's fuselage.
Note the large differ-
ence between top-side
temperatures and
those on the bottom of
the fuselage. (NASA)
H£SO FUSELAGE SKIN TEMPERATURES
SPEED DESIGN MISSION— TOP ^
I0i
800
600
400
200
.06 M .045 .050 an
I SKIN] I I
-S4GE _4 JL,
)72 IN. .062 .062j04
IMM1 FUSELAGE DESIGN TEMPERATURES
460°F
1240'F I230 F 620'F
*****
,I200°F
I200'F
1200*F
I240°F 1200' F 1200' F
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
25
X'15 Design and Development
Chapter 2
of the programmed flight plan. A program
utilizing a free-flying model had proved low-
speed stability and control to be adequate.
Since some aerodynamicists had questioned
North American's use of the rolling-tail
instead of ailerons, the free-flying model had
also been used to investigate that feature.
The results indicated that the rolling-tail
would provide the necessary lateral control.
Several papers presented at the conference
dealt with aerodynamic heating. One of these
was a summary of the experience gained with
the Bell X-1B and X-2. The information was
incomplete and not fully applicable to the
X-15, but it did provide a basis for compari-
son with the results of the wind tunnel and
analytical studies. Another paper dealt with
the results of the structural temperature esti-
mates that had been arrived at analytically. It
was apparent from the contents of the papers
that the engineers compiling them were con-
fronted by a paradox — in order to attain an
adequate and reasonably safe research vehi-
cle, they had to foresee and compensate for
the very aerodynamic heating problems that
were to be explored by the completed aircraft.
In addition to the papers on the theoretical
aspects of aerodynamic heating, a report was
made on the structural design that had been
accomplished at the time of the conference.
Critical loads would be encountered during
the accelerations at launch weight and during
reentry into the atmosphere, but since maxi-
mum temperatures would be encountered
only during the latter, the paper was largely
confined to the results of the investigations
of the load-temperature relationships that
were anticipated for the reentry phase. The
selection of Inconel X skin for the multispar
box-beam wing was justified on the basis of
the strength and favorable creep characteris-
tics of that material at 1,200 degrees
Fahrenheit. A milled bar of Inconel X was to
be used for the leading edge since that por-
tion of the wing acted as a heat sink. The
internal structure of the wing was to be of
titanium-alloy sheet and extrusion construc-
tion. The front and rear spars were to be flat
web-channel sections with the intermediate
spars and ribs of corrugated titanium webs.
For purposes of the tests the maximum tem-
perature differences between the upper and
lower wing surfaces had been estimated to be
400 degrees Fahrenheit and that between the
skin and the center of the spar as 960 degrees
Fahrenheit. Laboratory tests indicated that
such differences could be tolerated without
any adverse effects on the structure. Other
tests had proven that thermal stresses for the
Inconel-titanium structure were less than
those encountered in similar structures con-
structed entirely on Inconel X. Full-scale
tests had been made to determine the effects
of temperature on the buckling and ultimate
strength of a box beam. Simply heating the
test structure produced no surface buckles.
Compression buckles had appeared when
ultimate loads were applied at normal tem-
peratures but the buckles disappeared with
the removal of the load. Tests at higher tem-
peratures and involving large temperature
X-15 WING
WING SUPPORTING STRUCTURE
The wing of the
X-1 5 was constructed
from Inconel X skins
over a titanium struc-
ture. Unlike many air-
craft, there was not a
continuous spar
across both wings.
Instead, each wing
was bolted to the
fuselage. (NASA)
26
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 2
X-15 Design and Development
differences had finally led to the failure of
the test box, but it seemed safe to conclude
that ". . . thermal stresses had very little effect
on the ultimate strength of the box."
Tests similar to those conducted on the wing
structure had also been performed on the hor-
izontal stabilizer. The planned stabilizer struc-
ture differed from the wing in that it incorpo-
rated a stainless steel spar about halfway
between the leading and trailing edges, and an
Inconel X spar three and one-half inches from
the leading edge. The remainder of the inter-
nal structure consisted of titanium compo-
nents and the skin was Inconel X sheet. Tests
of the stabilizer had indicated that a design
which would prevent all skin buckling would
be inordinately heavy, so engineers decided to
tolerate temporary buckles. The proposed sta-
bilizer had flutter characteristics that were
within acceptable limits.
The front and rear fuselage were semimono-
coque structures of titanium ribs, Inconel X
outer skin, and an inner aluminum skin insu-
lated with spun glass. The integral propellant
tanks in the center fuselage were of full
monocoque construction. The full mono-
coque design used only slightiy thicker skins
than the semimonocoque design, possessed
adequate heat sink properties, and reduced
stresses caused by temperature differences by
placing all of the material at the surface. It
seemed, therefore, that the resulting structure
was ideal for use as a pressure tank. The
thickness of the monocoque walls would also
make sealing easier and leaks less likely.
The fuselage side tunnels presented yet
another problem. As the tunnels would pro-
tect the side portions of the propellant tanks
from aerodynamic heating, the sides would
not expand as rapidly as the areas exposed to
the air, and another undesirable compressive
stress had to be anticipated. It was thought
that beading the skin of the areas protected
by the tunnels would provide a satisfactory
solution, but beading introduced further
complications by reducing the structure's
ability to carry pressure loads. Ultimately,
however, the techniques proved successful.
Like most rocket engines of the period, the
XLR99 would use liquid oxygen as an oxi-
dizer, and a non-cryogenic fuel, in this case
anhydrous ammonia. 14 Each of the two main
propellant tanks was to be divided into three
compartments by curved bulkheads; the two
compartments furthest from the aircraft cen-
ter of gravity were equipped with slosh baf-
fles. Plumbing was to be installed in a single
compartment, the compartment sealed by a
bulkhead, and the process repeated until all
the compartments were completed. The tank
ends were to be semicurved in shape to keep
them as flat as possible, to reduce weight,
and to permit thermal expansion of the tank
shell. This entire structure was to be of weld-
ed Inconel X.
The expected acceleration of the X-15 pre-
sented several unique human factors concerns
early in the program. It was expected that the
pilot would be subjected to an acceleration of
up to 5g. It seemed advisable to develop a
One of the innovations
proposed by North
American was the use
of monocoque propel-
lant tanks, leading to
the use of the contro-
versial fuselage tun-
nels. The forward-most
part of the LOX tank
was equipped with
slosh baffles. (NASA)
FUSELAGE MAIN SHELL
MONOCOQUE
SHELL
THERMAL
STRAIN
RELIEF
BEADS
WING
SUPPORT
FRAME
TUNNEL
ATTACH
ANGLE
WING ATTACH FITTING
LIQUID OXYGEN TANKS
SLOSH
BAFFLE
-CLOSE-OUT WELD
BAFFLE TORUS
CENTER TUBE
STIFFENING RING
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
27
X-15 Design and Development
Chapter 2
side-stick controller that would allow the
pilot's arm to be supported by an armrest
while still allowing him of full control over
the aircraft. 15 Coupled with the fact that there
were two separate attitude-control systems on
the X-15, this resulted in a unique control
stick arrangement. A conventional center
stick, similar to that installed in most fighter-
type aircraft of the era, was connected to the
aerodynamic control surfaces through a sta-
bility-augmentation (damper) system. A side-
stick controller on the right console was con-
nected to the same aerodynamic control sur-
faces and augmentation system. Either stick
could be used interchangeably, although the
flight manual 16 describes using the center stick
"during normal periods of longitudinal and
vertical acceleration." The center stick was
occasionally omitted from flights later in the
flight research program based on pilot prefer-
ences. Another side-stick controller on the left
console operated the so-called "ballistic con-
trol" system 17 (thrusters) that provided attitude
control at high altitudes. The flight manual
warns that "velocity tends to sustain itself
after the stick is returned to the neutral posi-
tion. A subsequent stick movement opposite
to the initial one is required to cancel the orig-
inal attitude change."
At the time of the industry conference in
1956, the design for the X-15 side controller
had not been definitely established but a
summary of the previous experience with
such controllers was available. Experimental
controllers had been installed on a Grumman
F9F-2, Lockheed TV-2, Convair F-102, and
on a simulator. The pilots who had tried side
controllers had reported no difficulty in
maneuvering, but they generally felt that
greater efforts would have to be made to
eliminate backlash and to control friction
forces; they had also urged that efforts be
made to give the side controllers a more
"natural" feel.
Another problem which had not been thor-
oughly explored at the time of the 1956 con-
ference concerned the proposed reaction con-
trols that would be necessary for the X-15 as
dynamic pressures decreased to the point
where the aerodynamic controls would no
longer be effective. Analog computer and
ground simulator studies were then under way
in an effort to determine the best relationship
between the control thrust and the pilot's
movement of the control stick. Attempts were
also being made to determine the amount of
propellant that would be required for the reac-
3S2Q CONSOlf AERODYNAMIC CONTROL
PITCH THROW
WRIST RESTRAINED
6 S - IO°TO-35°
STICK REF POINT
PITCH AND ROLL
ENVELOPE OF
MOTION
ROLL AXIS
NOSE DOWN LIMIT
The X-15 contained
two side-stick con-
trollers; one for the
aerodynamic controls
(shown), and one on
the other console for
the reaction controls.
Although the side-stick
proved very success-
ful on the X-15, it
would be another 20
years before one was
installed on an opera-
tional aircraft (the
General Dynamics
F-16).(NASA)
28
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 2
X'15 Design and Development
Although the ejection
seat showed at the
1956 industry confer-
ence did not resemble
the final unit used in
the X-1 5s, the basic
concepts remained
the same. Restraining
the pilot's head, arms,
and legs during ejec-
tion at high dynamic
pressures presented
one of the major chal-
lenges to seat devel-
opment. (NASA)
tion controls. No significant problems were
uncovered during these early investigations,
but it was clear that the pilot would have to
give almost constant attention to such a con-
trol system and that pilots should be given
extensive practice on simulators before being
allowed to attempt actual flight.
Some of the anticipated difficulties in the field
of instrumentation arose because available
strain gauges were not considered satisfactory
at the expected high temperatures and because
of difficulties in recording the output of ther-
mocouples. Large structural deformations of
wings and empennage were to be recorded by
cameras in special camera compartments.
Another instrumentation problem arose
because the sensing of static pressure, ordi-
narily difficult at high Mach numbers, was
compounded in the case of the X-15 by heat-
ing that would be too great for any conven-
tional probe and by the low pressure at the
high altitudes to be explored. The answer was
to develop a stable-platform-integrating-
accelerometer system to provide velocity, alti-
tude, pitch, yaw, and roll angle information.
Still another instrumentation difficulty was
created by the desirability of presenting the
pilot with angle-of-attack and side slip infor-
mation, especially for the critical exit and
reentry periods. Any device to furnish this
information would have to be located ahead
of the aircraft's own flow disturbances, be
structurally sound at elevated temperatures,
accurate at low pressures, and cause a mini-
mal flow disturbance so as not to interfere
with the heat transfer studies that were to be
conducted in the forward area of the fuse-
lage. These requirements had resulted in the
design of a ball-nose 18 capable of withstand-
ing 1,200 degrees Fahrenheit. A six-inch
diameter Inconel X sphere located in the
extreme nose of the X-15 was gimbaled 19 and
servo-driven in two planes. It had five open-
ings: a total-head port opening directly for-
ward and two pairs of angle-sensing ports in
the pitch and yaw planes, located at an angle
of 30 to 40 degrees from the central port.
Pitch and yaw could be sensed as pressure
differences and these differences were con-
verted into signals that would cause the ser-
vos to realign the sphere in the relative wind.
Based largely on urgings from Scott
Crossfield, the Air Force agreed to allow
North American to design an ejection seat
and to make a study justifying the selection
m^n EJECTION SEAT RELEASE SYSTEM
AUTOMATIC
SEPARATION
PILOT IS RESTRAINED
IN THE EJECTION SEAT*.
AT THE HIPS
AT THE SHOULDERS
AT THE FEET
AT THE HEAD
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
29
X-15 Design and Development
Chapter 2
of a seat in preference to a capsule system. 20
Two main criteria had governed the selec-
tion of an escape system for the X-15, and
these criteria were not necessarily comple-
mentary. The first requirement was that the
system be the most suitable that could be
designed while remaining compatible with
the airplane. The second was that no system
would be selected that would delay the
development of the X-15 or leave the pilot
without any method of escape when the
time arrived for flight research. The four
possible escape systems that were consid-
ered included cockpit capsules, nose cap-
sules, a canopy shielded seat, and a stable-
seat with a pressure-suit. An analysis of the
expected flight hazards had indicated that
because of the fuel exhaustion and low
aerodynamic loads, the accident potential at
peak speeds and altitudes was only about
two percent of the total.
Capsule-like systems had been tried before,
most notably in the X-2 where the entire for-
ward fuselage could be detached from the rest
of the aircraft. Model tests showed these to be
very unstable and prone to tumble at a high
rate of rotation. They also added a great deal
of weight and complexity to the aircraft. 21
The final decision for a stable-seat with a
pressure-suit was made because most of the
potential accidents could be expected to
occur at speeds of Mach 4 or less, because
system reliability always decreased with sys-
tem complexity, and finally, because it was
the system that imposed the smallest weight
and size penalties upon the aircraft. The
selected system would not function success-
fully at altitudes above 120,000 feet or speeds
in excess of Mach 4, but designers, particu-
larly Scott Crossfield, held that the aircraft
itself would offer the best protection in the
areas of the performance envelope where the
seat-suit combination was inadequate.
Cockpit and instrument cooling, pressuriza-
tion, suit ventilation, windshield defogging,
and fire protection were all to be provided
from a liquid nitrogen supply. Vaporization
of the liquid nitrogen would keep the pilot's
environment within comfortable limits at all
times. An interesting aspect of the cooling
problem was an estimate that only 1.5 per-
cent of the system's capacity would be
applied to the pilot; the remaining 98.5 per-
cent was required for equipment. Cockpit
temperatures were to be limited to no more
than 150 degrees Fahrenheit, the maximum
12I*±ZU ANALYSIS OF X-15 ACCIDENT POTENTIAL
tet\
£OUf
-«•«
ALT
IOOOFT
0.5%* ;
/ ' 25
200
160
120
- — 7 L__ — 50
V- — -~\*»ioo
1.5% -^ s ""l
... . A VB.0.
6 %..--\ \ X
80
x--** -- \ \ **i5ocN
92% „*- \ \ ^~~-~^~~^ZZZ~
40
^<^S^^*^ ^-EXPECTED
S^**^- DESIGN LIMITS FLIGHT LIMITS
u (
) 1.0 2.0 3.0 4.0 5.0 ^ 6.0 7.0
This chart shows that
92 percent of the
expected X-15 acci-
dents would happen
below Mach 2 and
90,000 feet. This esti-
mate supported Scott
Crossfield's request to
use an ejection seat
and pressure suit
instead of a more
complex escape cap-
sule. (NASA)
30
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 2
X-15 Design and Development
Despite its perform-
ance potential, the
basic cockpit design
of the X-15 was quite
conventional, with the
exception of the side-
stick controllers. The
engine instrumenta-
tion on the lower left
of the instrument
panel would be differ-
ent for the XLR1 1
flights. The addition of
the MH-96 in the
X-15-3 would necessi-
tate some changes in
the instrumentation.
See page 63 for a
photo. (NASA)
limit for some of the equipment. The pilot
would not be subjected to that temperature,
however, as the pressure suit ventilation
would enable him to select a comfortable
temperature level. Cockpit pressure was to
be maintained at the 35,000 foot level.
The effects of flight accelerations upon the
pilot's physiological condition and upon his
ability to avoid inadvertent control move-
ments had not been completely explored,
but it was recognized that high accelera-
tions could pose medical and restraint diffi-
culties. In addition to the accelerations that
would be encountered during the exit and
reentry phases of the X-15's flights, a very
high acceleration of short duration would
be produced during the landings. This was a
result of the location of the main skids at
the rear of the aircraft. Once the skids
touched down, the entire aircraft would act
as if it were hinged at the skid attachment
points and the nose section would slam
downward. Reproduction of this landing
acceleration on simulators showed that
because of the short duration, no real prob-
lem existed. There were, however, numer-
ous complaints about the severity of the
jolts both in the simulator and once actual
landings began.
The final paper presented to the 1956 indus-
try conference was an excellent summary of
the development effort and a review of the
major problems that were known at that
time. The author, Lawrence P. Greene from
North American, considered flutter to be an
unsolved problem, primarily because of a
lack of basic data on aero-thermal-elastic
relationships and because little experimental
data was available on flutter at hypersonic
Mach numbers. He pointed out that available
data on high-speed flutter had been derived
from experiments conducted at Mach 3 or
less, and that not all of the data obtained at
those speeds were applicable to the problems
faced by the designers of the X-15. As it
turned out, panel flutter was encountered
early in the flight test program, leading to a
change in the design criteria for high-speed
aircraft. Another difficulty was the newness
of Inconel X as a structural material and the
necessity of experimenting with fabrication
techniques that would permit its use as the
primary structural material for the X-15.
Problems were also expected to arise in con-
nection with sealing materials, most of
which were known to react unfavorably
when subjected to high temperature condi-
tions. 22 Although North American did
encounter initial problems in using Inconel
ISYS-447U
X-15 COCKPIT
ENGINE
EN6 CONTROLS-
SPACE
ATTITUDE
CONTROL-
FLIGHT
-APU % ELEC
-CONSOLE
AERO
CONTROL
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
31
X-J5 Design and Development
Chapter 2
X and titanium during the construction of the
X-15, it was able to work through the diffi-
culties with no major delays.
A development engineering inspection was
held at the North American Inglewood plant
on 12-13 December 1956. This inspection
of a full-scale mockup was intended to
reveal unsatisfactory design features before
fabrication of the aircraft got under way.
Thirty-four of the forty-nine individuals
who participated in the inspection were rep-
resentatives of the Air Force; twenty-two of
them from WADC. The important role of
the Air Force was also evident from the
composition of the committee that would
review the requests for alteration. 23 Major E.
C. Freeman, of ARDC, served as committee
chairman, Mr. F. Orazio of WADC and
Lieutenant Colonel Keith G. Lindell of Air
Force Headquarters were committee mem-
bers, and Captain Chester E. McCollough,
Jr. of the ARDC and Captain Iven C.
Kincheloe, Jr. of the Air Force Flight Test
Center (AFFTC) served as advisors. The
Navy and the NACA each provided a single
committee member; three additional advi-
sors were drawn from the NACA.
The inspection committee considered 84'
requests for alterations, rejected 12, and
placed 22 in a category for further study. The
majority of the 50 changes that were accept-
ed were minor, such as the addition of longi-
tudinal trim indications from the stick posi-
tion and trim switches, relocation of the bat-
tery switch, removal of landing gear warning
lights, rearrangement and redesign of warn-
ing lights, and improved markings for sever-
al instruments and controls.
Some of the most interesting comments were
rejected by the committee. For instance, the
suggestions that the aerodynamic and reac-
tion controller motions be made similar, that
the reaction controls be made operable by
the same controller used for the aerodynam-
ic controls, or that a third controller combin-
ing the functions of the aerodynamic and
reaction controllers be added to the right
console, were all rejected on the grounds that
actual flight experience was needed with the
controllers already selected before a decision
could be made on worthwhile improvements
or combinations. As two of the three sugges-
tions on the controllers came from potential
pilots of the X-15 (Joseph A. Walker and
The vertical stabilizer
was one of the most
obvious changes
between the industry
conference configura-
tion and the final vehi-
cle. The first design
did not use the exag-
gerated wedge-shape
of the final unit. It was
also more traditional,
using a fixed forward
portion and a conven-
tional appearing rud-
der. The final version
used an all-moving
design. Note the rud-
der splits to become
speed brakes, much
like the shuttle design
25 years later. (NASA)
32
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 2
X'I5 Design and Development
Iven C. Kincheloe, Jr. 24 ), it would appear that
the planned controllers were not all that
might have been desired.
A request that the pilot be provided with
continuous information on the nose-wheel
door position (loss of the door could produce
severe structural damage) was rejected
because the committee felt that the previous-
ly approved suggestion for gear-up inspec-
tion panels would make such information
unnecessary. This particular item would
come back to haunt the program during the
flight research phase.
After the completion of the development
engineering inspection, the X-15 airframe
design changed only in relatively minor
details. North American essentially built the
X-15 described at the industry conference in
October and inspected in mockup in
December 1956. Continued wind tunnel test-
ing resulted in some external modifications,
particularly of the vertical stabilizer, and
some weight changes occurred as plans
became more definite. But while work on the
airframe progressed smoothly, with few
unexpected problems, the project as a whole
did encounter difficulties, some of them seri-
ous enough to threaten long delays. In fact,
North American's rapid preparation of draw-
ings and production planning served to high-
light the lack of progress on some of the
components and subsystems that were essen-
tial to the success of the program.
The Engine
Those concerned with the success of the X-15
had to monitor the development of the aircraft
itself, the XLR99 rocket engine, the auxiliary
power units, an inertial system, a tracking
range, a pressure suit, and an ejection seat.
They had to make arrangements for support
and B-36 carrier aircraft, ground equipment,
the selection of pilots, and the development of
simulators for pilot training. It was necessary
to secure time on centrifuges, in wind tunnels,
and on sled tracks. The ball-nose had to be
developed, studies made of the compatibility
of the X-15 and the carrier aircraft, and other
studies on the possibility of extending the
X-15 program beyond the goals originally
contemplated. In addition to such tasks, funds
to cover ever increasing costs had to be
secured if the project were to have any chance
of ultimate success, and at certain stages the
effects of possibly harmful publicity had to be
considered. With such multiplicity of tasks, it
could be expected that several serious prob-
lems would arise; not surprisingly, probably
the most serious arose during the develop-
ment of the XLR99.
Finding a suitable engine for the X-15 had
been somewhat problematic from the earliest
stages of the project, when the WADC Power
Plant Laboratory had pointed out that the lack
of an acceptable rocket engine was the major
shortcoming of the NACA's original propos-
al. The laboratory did not believe that any
available engine was entirely suitable for the
X-15 and held that no matter what engine
was accepted, a considerable amount of
development work could be anticipated. Most
of the possible engines were either too small
or would need too long a development peri-
od. In spite of these reservations, the labora-
tory listed a number of engines worth consid-
ering and drew up a statement of the require-
ments for an engine that would be suitable for
the proposed X-15 design. The laboratory
also made clear its stand that the government
should "... accept responsibility for develop-
ment of the selected engine and . . . provide
this engine to the airplane contractor as
Government Furnished Equipment." 25
The primary requirement for an X-15
engine, as outlined in 1954, was that it be
capable of operating safely under all condi-
tions. Service life would not have to be as
long as for a production engine, but engi-
neers hoped that the selected engine would
not depart too far from production standards.
The same attitude was taken toward reliabil-
ity; the engine need not be as reliable as a
production article, but it should approach
such reliability as nearly as possible. There
could be no altitude limitations for starting
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
33
X-15 Design and Development
Chapter 2
or operating the engine, and the power plant
would have to be entirely safe during start,
operation, and shutdown, no matter what the
altitude. The laboratory made it quite clear
that a variable thrust engine capable of
repeated restarts was essential.
The engine ultimately selected was not one
of the four originally presented as possibil-
ities by the Power Plant Laboratory. The
ultimate selection was foreshadowed, how-
ever, in discussions with Reaction Motors
concerning the XLR10, during which atten-
tion was drawn to what was termed "... a
larger version of [the] Viking engine
[XLR30]." In light of subsequent events, it
was interesting to note that the laboratory
thought 26 the XLR30 could be developed
into a suitable X-15 engine for "... less than
$5,000,000 ..." and with" ... approximate-
ly two years' work." 27
After North American had been selected as
the winner of the X-15 competition, plans
were instituted to procure the modified
XLR30 engine that had been incorporated in
the winning design. Late in October,
Reaction Motors was notified that North
American had won the X-15 competition and
that the winner had based his proposals upon
the XLR30 engine. 23
On 1 December 1955 a $1,000,000 letter con-
tract was initiated with Reaction Motors for
the development of a rocket engine for the
X-15. 29 Soon afterwards, a controversy devel-
oped over the assignment of cognizance for
the development of the engine. It began with
a letter from Rear Adm. W. A. Schoech of the
Bureau of Aeronautics. Adm. Schoech con-
tended that since the XLR30-RM-2 rocket
engine was the basis for the X-15 power
plant, and the BuAer had already devoted
three years to the development of that engine,
it would be logical to assign the responsibili-
ty for further development to the Navy. The
admiral felt that retention of the program by
the BuAer would expedite development,
especially as the Navy could direct the devel-
opment toward an X-15 engine by making
specification changes rather than by negotiat-
ing a new contract. 30
The Navy's bid for control of the engine
development was rejected on 3 January 1956
on the grounds that the management respon-
sibility should be vested in a single agency,
that conflict of interest might generate delay,
HI±2a POCKET- ENGINE INSTALLATION
521
565 ENGINE MOUNT
565
ACCESS DOORS
AFT FAIRING
The XLR99 was an
extremely compact
engine, considering it
was able to produce
over 57,000 pounds-
thrust. This was the
first throttleable and
restartable man-rated
rocket engine. Many of
the lessons-learned
from this engine were
incorporated into the
Space Shuttle Main
Engine developed 20
years later. (NASA)
34
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 2
X'l5 Design and Development
This 1956 sketch
shows the controls
and indicators for the
XLR99. A different set
of controls were used
for the XLR1 1 flights,
although they fit into
the same space allo-
cation. Notice the sim-
ple throttle on the left
console, underneath
the reaction control
side-stick (not shown).
The jettison controls
took on particular sig-
nificance on missions
that had to be aborted
prior to engine burn-
out. (NASA)
and that BuAer was underestimating the time
and effort necessary to make the XLR30 a
satisfactory engine for piloted flight.
The final Reaction Motors technical propos-
al was received by the Power Plant
Laboratory on 24 January, with the cost pro-
posal following on 8 February. 31 The cover
letter from Reaction Motors promised deliv-
ery of the first complete system "..., within
thirty (30) months after we are authorized to
proceed." 32 Reaction Motors also estimated
that the entire cost of the program would
total $10,480,718. 33 On 21 February the new
engine was designated XLR99-RM-1. 34
The 1956 industry conference heard two
papers on the proposed engine and propul-
sion system for the X-15. The XLR99-RM-1
would be able to vary its thrust from 19,200
to 57,200 pounds at 40,000 feet using anhy-
drous ammonia and liquid oxygen (LOX) 35
as propellants. Specific impulse was to vary
from a minimum of 256 seconds to a maxi-
mum of 276 seconds. The engine was to fit
into a space 71.7 inches long and 43.2 inch-
es in diameter, have a dry weight of 618
pounds, and a wet weight of 748 pounds. A
single thrust chamber was supplied by a
hydrogen-peroxide-driven turbopump, with
the turbopump's exhaust being recovered in
the thrust chamber. Thrust control was by
regulation of the turbopump speed. 36
The use of ammonia as a propellant present-
ed some potential problems; in addition to
being toxic in high concentrations, ammonia
is also corrosive to all copper-based metals.
There were discussions early in the program
between the Air Force, Reaction Motors, and
the Lewis Research Center 37 about the possi-
bility of switching to a hydrocarbon fuel. It
was finally concluded that changing fuel
would add six months to the development
schedule; it would be easier to learn to live
with the ammonia. 38 There is no documenta-
tion that the ammonia ultimately presented
any significant problems to the program.
The decision to control thrust by regulating
the speed of the turbopump was made
because the other possibilities (regulation by
measurement of the pressure in the thrust
chamber or of the pressure of the discharge)
would cause the turbopump to speed up as
pressure dropped. As the most likely cause of
pressure drop would be cavitation in the pro-
pellant system, an increase in turbopump
Wml PROPULSfON-SYSTEM PILOT CONTROLS
INSTRUMENT PANEL-
TANK PRESSURE $
JETTISON CONTROLS
ST0PJETT.
ENGINE INDICATOR
LIGHTS
■ENGINE CONTROL
SWITCHES
-CHAMBER
PRESSURE
-TOTAL
IMPULSE
REMAINING
-THROTTLE
PR0PELLANT-5YSTEM 4
PURGE-SYSTEM PRESSURES
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
35
X'15 Design and Development
Chapter 2
speed would aggravate rather than correct
the situation. Reaction Motors had also
decided that varying the injection area was
too complicated a method for attaining a
variable thrust engine and had chosen to vary
the injection pressure instead.
The regenerative cooling of the thrust cham-
ber created another problem since the vari-
able fuel flow of a throttleable engine meant
that the system's cooling capacity would also
vary and that adequate cooling throughout
the engine's operating range would produce
excessive cooling under some conditions.
Engine compartment temperatures also had
to be given more consideration than in previ-
ous rocket engine designs because of the
higher radiant heat transfer from the struc-
ture of the X-15. Reaction Motors'
spokesman at the 1956 industry conference
concluded that the development of the
XLR99 was going to be a difficult task.
Subsequent events were certainly to prove
the validity of that prediction.
A second paper dealt with engine and acces-
sory installation, the location of the propel-
lant system components, and the engine con-
trols and instruments. The main propellant
tanks were to contain the LOX, ammonia,
and the hydrogen peroxide. The LOX tank,
with a capacity of approximately 1,000 gal-
lons, was located just ahead of the aircraft's
center of gravity; the 1,400 gallon ammonia
tank was just aft of the same point. A center
core tube within the LOX tank would pro-
vide a location for a supply of helium under
a pressure of 3,600 psi. Helium was used to
pressurize both the LOX and amimonia tanks.
A ?5-gallon hydrogen peroxide tank behind
the ammonia tank provided the monopropel-
lant for the turbopump.
Provision was also made to top-off the LOX
tank from a supply carried aboard the carrier
aircraft; this was considered to be beneficial
in two ways. The LOX supply in the carrier
aircraft could be kept cooler than the oxygen
already aboard the X-15, and the added LOX
would permit cooling of the X-15's own sup-
ply by boil-off, without reduction of the
quantity available for flight. The ammonia
tank was not to be provided with a top-off
arrangement, as the slight increase in fuel
temperature during flight was not considered
significant enough to justify the complica-
tions such a system would have entailed.
On 10 July 1957, Reaction Motors advised
the Air Force that an engine satisfying the
contract specifications could not be devel-
oped unless the government agreed to a nine-
The XLR99 on a main-
tenance stand. The
engine used ammonia
(NH3) as fuel and liq-
uid oxygen (LOX) as
the oxidizer. The
XLR99 required a sep-
arate propellant, hydro-
gen peroxide, to drive
its high-speed turbop-
ump — the Space
Shuttle Main Engine
uses the propellant
itself (LH2 or L.02, as
appropriate) to drive
the turbopumps.
(AFFTC via the Tony
Landis Collection)
36
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 2
%'15 Design and Development
month schedule extension and a cost increase
from $15,000,000 to $21,800,000. At the
same time, Reaction Motors indicated that it
could provide an engine that met the per-
formance specification within the established
schedule if permitted to increase the weight
from 618 pounds to 836 pounds. The compa-
ny estimated that this overweight engine
could be provided for $17,100,000. The Air
Force elected to pursue the heavier engine
since it would be available sooner and have
less impact on the overall X-15 program.
Those who hoped that the overall perform-
ance of the X-15 would be maintained were
encouraged by a report that the turbopump
was more efficient than anticipated and
would allow a 197 pound reduction in the
amount of hydrogen peroxide necessary for
its operation. This decrease, a lighter than
expected airframe, and the increase in launch
speeds and altitudes provided by a recent
substitution of a B-52 as the carrier aircraft,
offered some hope that the original X-15 per-
formance goals might still be achieved. 39
Despite the relaxation of the weight require-
ments, the engine program failed to proceed
at a satisfactory pace. On 1 1 December 1957
Reaction Motors reported a new six-month
slip. The threat to the entire X-15 program
posed by these new delays was a matter of
serious concern, and on 7 January 1958,
Reaction Motors was asked to furnish a
detailed schedule and to propose means for
solving the difficulties. The new schedule,
which reached WADC in mid- January, indi-
cated that the program would be delayed
another five and one-half months and that
costs would rise to $34,400,000— double the
cost estimate of the previous July. 40
In reaction, the Air Force recommended
increasing the resources available to
Reaction Motors and proposed the use of
two 41 XLR11 rocket engines as an interim
installation for the initial X-15 flights.
Additional funds to cover the increased
effort were also approved, as was the estab-
lishment of an advisory group. 42
The threat that engine delays would serious-
ly impair the value of the X-15 program had
generated a whole series of actions during
the first half of 1958: personal visits by gen-
eral officers to Reaction Motors, numerous
conferences between the contractor and
representatives of government agencies,
increased support from the Propulsion
Laboratory 43 and the NACA, an increase in
funds, and letters containing severe censure
of the company's conduct of the program. An
emergency situation had been encountered,
emergency remedies were used, and by mid-
summer improvements began to be noted.
Engine progress continued to be reasonably
satisfactory during the remainder of 1958. A
destructive failure that occurred on 24
October was traced to components that had
already been recognized as inadequate and
were in the process of being redesigned. The
failure, therefore, was not considered of major
importance. 44 A long-sought goal was finally
reached on 18 April 1959 with completion of
the Preliminary Flight Rating Test (PFRT).
The flight rating program began at once. 45
At the end of April, a "realistic" schedule for
the remainder of the program showed that
the Flight Rating Test would be completed
by 1 September 1959. The first ground test
engine was delivered to Edwards AFB at the
end of May, and the first flight engine was
delivered at the end of July. 46
A total of 10 flight engines were initially
procured, along with six spare injector-
chamber assemblies; one additional flight
engine was subsequently procured. In
January 1961, shortly after the first XLR99
test flight, only eight of these engines were
available to the flight test program. There
was still a number of problems with the
engines that Reaction Motors was continuing
to work on; the most serious being a vibra-
tion at certain power levels, and a shorter
than expected chamber life. There were four
engines being used for continued ground
tests, including two flight engines. 47 Three of
the engines were involved in tests to isolate
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
37
X'15 Design, and Development
Chapter 2
and eliminate the vibrations, while the fourth
engine was being used to investigate extend-
ing the life of the chamber. 48
It is interesting to note that early in the pro-
posal stage, North American determined that
aerodynamic drag was not as important a
design factor as was normally the case with
jet-powered fighters. This was largely due to
the amount of excess thrust available from
the XLR99. Weight was considered a larger
driver in the overall airplane design. Only
about 10 percent of the total engine thrust
was necessary to overcome drag, and anoth-
er 20 percent to overcome weight. The
remaining 70 percent of engine thrust was
available to accelerate the X-15. 49
Other Systems
In early 1958, at the very height of the furor
over the problems with the XLR99, a note of
warning sounded for the General Electric
auxiliary power unit (APU). On 26 March
1958 and again on 11 April 1958, General
Electric notified North American of its
inability to meet the original specifications
in the time available, and requested approval
of new specifications. North American, with
the concurrence of the Air Force, agreed to
modify the requirements. The major changes
involved an increase in weight from 40 to 48
pounds, an increase in start time from five to
seven seconds, and a revision of the specific
fuel consumption curves. 50
By the end of the summer 1958, the APU
seemed to have reached a more satisfactory
state of development, and production units
were ready for shipment. 51 The early captive
flights beginning in 1959 would reveal some
additional problems, but investigation showed
that the in-flight failures had occurred partial-
ly because captive testing subjected the units
to an abnormal operational sequence that
would not be encountered during glide and
powered flight. Some components were
redesigned, but the APU would continue to be
relatively troublesome in actual service.
During the course of the X-15 program, many
concerns were voiced over the development
of a pressure suit and an escape system.
Although full-pressure suits had been studied
during World War n, attempts to fabricate a
practical garment had met with failure. The
mm INFLUENCE OF WEIGHT AND DRAG
60
t
THRUST, DRAG $
WEIGHT-I000 LB
V
THRUST
THRUST USED TO ACCELERATE
DRAG PLUS WEIGHT
COMPONENT
20 30 40 50 60
TIME
SEC
80 90
Soule to Storms: "You
have a little airplane
and a big engine with
a large thrust margin."
And indeed they did.
The XLR99 provided
57,000 pounds-thrust
to propel an aircraft
that only weighed
30,000 pounds.
Consider that the con-
temporary F-1 04
Starfighter, considered
something of a hot
rod, weighed 20,000
pounds and its J79
only produced 15,000
pounds-thrust in full
afterburner. (NASA)
38
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 2
X'15 Design and Development
Air Force took renewed interest in pressure
suits in 1954 when it had become obvious
that the increasing performance of aircraft
was going to necessitate such a garment. The
first result of the renewed interest was the
creation of a suit that was heavy, bulky, and
unwieldy; the garment had only limited
mobility and various joints created painful
pressure points. However, in 1955 the David
Clark Company succeeded in producing a
garment using a distorted-angle fabric that
held some promise of ultimate success. 52
Despite the early state-of-development of
full-pressure suits, Scott Crossfield was con-
vinced they were the way to go for X-15.
North American's detail specifications of 2
March 1956 called for just such a garment —
to be furnished by North American through a
subcontract with the David Clark Company. 53
A positive step toward Air Force acceptance
of the idea occurred during a conference held
at the North American plant on 20-22 June
1956. A full-pressure suit developed by the
Navy was demonstrated during an inspection
of the preliminary cockpit mockup, and
although the suit still had a number of defi-
ciencies, it was concluded that "... the state-
of-the-art on full pressure suits should permit
the development of such a suit satisfactory
for use in the X-15." 54
After an extremely difficult and prolonged
development process, Scott Crossfield
received the first new MC-2 full-pressure suit
on 17 December 1958 and, two days later, the
suit successfully passed nitrogen contamina-
tion tests at the Air Force Aero Medical
Laboratory. The X-15 project officer attrib-
uted much of the credit for the successful and
timely qualification of the full-pressure suit
to the intensive efforts of Crossfield. 55
Fortunately, development did not stop there.
On 27 July 1959, the Aero Medical
Laboratory brought the first of the new
A/P22S-2 pressure suits to Edwards. The
consensus amongst the pilots was that it rep-
resented a large improvement over the earli-
er MC-2. It was more comfortable and pro-
vided greater mobility; and it took only 5
minutes to put on, compared to 30 minutes
for the MC-2. However, it would take anoth-
er year before fully-qualified versions of the
suit were delivered to the X-15 program. 56
While not directly related to the pressure suit
difficulties, the type of escape system to be
used in the X-15 had been the subject of
debate at an early stage of the program; the
decision to use the stable-seat, full-pressure-
suit combination had been a compromise
based largely on the fact that the ejection seat
was lighter and offered fewer complications
than the other alternatives.
As early as 8 February 1955, the Aero
Medical Laboratory had recommended a cap-
sular escape system, but the laboratory had
also admitted that such a system would prob-
ably require extensive development. The sec-
ond choice was a stable seat that incorporated
limb retention features and one that would
produce a minimum of deceleration. 57 During
meetings held in October and November
1955, it was agreed that North American
would design an ejection seat for the X-15 and
would also prepare a report justifying the use
of such a system in preference to a capsule.
North American was to incorporate head and
limb restraints in the proposed seat. 58
Despite the report, the Air Force was not
completely convinced. At a meeting held at
Wright Field on 2-3 May 1956, the Air Force
again pointed out the limitations of ejection
seats. In the opinion of one NACA engineer
who attended the meeting, the Air Force was
still strongly in favor of a capsule — partly
because of the additional safety a capsule
system would offer, and partly because the
use of such a system in the X-15 would pro-
vide an opportunity for further developmen-
tal research. Primarily due to the efforts of
Scott Crossfield, the participants finally
agreed that because of the "time factor,
weight, ignorance about proper capsule
design, and the safety features being built
into the airplane structure itself, the X-15
was probably its own best capsule." About
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
39
X'15 Design and Development
Chapter 2
the only result of the reluctance of the Air
Force to endorse an ejection seat was a
request that North American yet again docu-
ment the arguments for the seat. 59
The death of Captain Milburn G. Apt in the
crash of the Bell X-2, which had been
equipped with an escape capsule, in
September 1956 renewed apprehension as to
the adequacy of the X-15's escape system. 60
By this time, however, it was acknowledged
that no substantive changes could be made to
the X-15 design. Fortunately, North
American's seat development efforts were
generally proceeding well. 61
Sled tests of the ejection seat began early in
1958 at Edwards with the preliminary tests
concluded on 22 April. Because of the high
cost of sled runs, the X-15 project office
advised North American to eliminate the
planned incremental testing and to conduct
the tests at just two pressure levels — 125
pounds per square foot and 1,500 pounds per
square foot. The X-15 office felt that suc-
cessful tests at these two levels would fur-
nish adequate proof of seat reliability at
intermediate pressures. 62
Between 4 June 1958 and 3 March 1959, the
X-15 seat completed its series of sled tests.
Various problems, with both the seat and the
sled, had been encountered, but all had been
worked through to the satisfaction of North
American and the Air Force. The X-15 seat
was cleared for flight use. 63
Another item for which the Air Force retained
direct responsibility was the all-attitude iner-
tial flight data system. It was realized from the
beginning of the X-15 program that the air-
plane's performance would necessitate a new
means of determining altitude, speed, and air-
craft attitude. This was because the traditional
use of static pressure as a reference would be
largely impossible at the speeds and altitudes
the X-15 would achieve; moreover, the tem-
peratures encountered would rule out the use
of tradition pitot tube sensing devices. The
NACA had proposed a "stable-platform iner-
tial-integrating and attitude sensing unit" as
the means of meeting these needs. 64 A series
of rmscommunications resulted in the NACA
assuming the Air Force had already developed
a satisfactory unit and would provide it to the
X-15 program. 65 After it was discovered that a
suitable unit did not exist, emergency efforts
were undertaken to develop one without
impacting the X-15 program. After a consid-
erable amount of controversy, a sole-source
contract was awarded to the Sperry
Gyroscope Company on 5 June 1957 for the
development and manufacture of the stable-
platform. 66 The cost-plus-fjxed-fee contract,
signed on 5 June 1957, was for $1,213,518.06
with a fixed fee of $85,000. 67
In April 1958, the Air Force concluded that
the scheduled delivery of the initial Sperry
unit in December would not permit adequate
testing to be performed prior to the first
flights of the X-15. Consequently a less capa-
ble interim gyroscopic system was installed
in the first two aircraft and the final Sperry
system was installed in the last X-15. 68
By the end of 1958, the two major system
components (the stabilizer and the computer)
were completed and ready to be tested as a
complete unit. The systems were shipped to
Edwards in late January 1959, and during the
spring of 1959 plans were made to use the
NB-52 carrier aircraft as a test vehicle. 69 In
addition, arrangements were made to test the
stable-platform in a KC-97 that was already
in use as a test aircraft in connection with the
B-58 program.™ The first test flights in the
KC-97 were carried out in late April. 71 By
June, North American had successfully
installed the Sperry system in the third
X-15. 72 In January 1961, wiring was installed
in the NB-52B to allow the stable-platform
to be installed in a pod carried on the pylon
under the wing. The first complete stable-
platform system installed in the B-52 pod
was flown on 1 March 1961. Since the B-52
was capable of greater speeds and higher
altitudes than the KC-97, it provided addi-
tional data to assist Sperry in resolving prob-
lems being encountered with the unit. 73
40
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 2
X-15 Design and Development
The use of a B-36 car-
rier aircraft would
have allowed the pilot
to exit the aircraft
while in transit to the
drop area, or in case
of emergency.
However, personnel at
the FRC worried that
the B-36 would not be
supportable since it
was being phased out
of active service. In
the end, the B-52 pro-
vided much better per-
formance and was
ultimately selected.
(AFFTC History
Office)
The High Range
Previous rocket aircraft, such as the X-l and
X-2, had been able to conduct the majority of
their flight research in the skies directly over
the Edwards test areas. The capabilities of the
X-15, however, would use vastly more air-
space. The proposed trajectories required an
essentially straight flight corridor equipped
with multiple tracking, telemetry, and com-
munications sites, as well as the need for suit-
able emergency landing areas. This led to con-
struction of the X-15 High Range extending
from Wendover, Utah, to Edwards AFB.
Radar and telemetry stations were installed at
Ely and Beatty, Nevada, as well as Edwards.
Telemetry from the X-15, as well as voice
communications, were received, recorded,
and forwarded to Edwards by the stations at
Ely and Beatty. Each of these stations was
also manned by a person to back up the prime
"communicator" (NASA 1) at Edwards in
case the communication links went down.
Each ground station overlapped the next, and
they were interconnected via microwave and
land-line so that timing signals, voice com-
munication, and radar data would be available
to all. Provisions were made for recording the
acquired data on tape and film, although some
of the data was directly displayed on strip and
plotting charts. The design and construction
of the range was accomplished by Electronic
Engineering Company of Los Angeles under
contract with the Air Force. 74 North American
and the NACA also conducted numerous
evaluations of various dry lakes to determine
which were suitable for emergency landings
along the route (see the summary included as
an appendix to this monograph).
Carrier Aircraft
The group at Langley had sized their X-15
proposal around the potential of using a
M*£U CARRIER INSTALLATION
CREW COMPARTMENT AND ACCESS TO X-15
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
41
X'15 Design and Development
Chapter 2
Convair B-36 as the carrier aircraft. This was a
natural extension of previous X-planes that
had used a Boeing B-29 or B-50 as a carrier.
The B-36 would be modified to carry the X-15
partially enclosed in its bomb bays, much like
the X-l and X-2 had been in earlier projects.
This arrangement had some advantages; the
pilot could freely move between the X-15 and
B-36 during climb-out and the cruise to the
launch location. This was extremely advanta-
geous if problems developed that required jet-
tisoning the X-15 prior to launch. At the time
of the first industry conference in 1956, it was
expected that a B-36 would be modified begin-
ning in the middle of 1957 and be ready for
flight tests in October 1958. 75
As the weight of the X-15 and its subsystems
grew, however, the Air Force and NASA
began to look for ways to recover some of the
lost performance. One way was to launch the
X-15 at a higher altitude and greater speed. In
addition, the personnel at Edwards believed
that the ten-engine B-36 would be difficult to
maintain 76 since it was being phased out of the
Air Force inventory. Investigations showed
that the X-15, as designed, would fit under
the wing of one of the new Boeing B-52
Stratofortresses; the configuration of the B-52
precluded carrying the X-15 in the bomb bay.
This was not the ideal solution— the X-15
pilot would have to be locked in the research
airplane prior to takeoff, and the large weight
transition when the X-15 was released would
provide some interesting control problems for
the B-52. Further analysis concluded that the
potential problems were solvable, and that the
increase in speed and altitude capabilities
were desirable. Fortunately, two early B-52s
were completing their test duties, and the Air
Force made them available to the program.
On 29 November 1957, the B-52A (52-003)
arrived at Air Force Plant 42 in Palmdale,
California, after a flight from the Boeing plant
in Seattle. The aircraft was placed in storage
pending modifications. On 4 February 1958,
the B-52A was moved into the North
American hanger at Plant 42 and modified
with a large pylon under the wing, the capa-
bility to monitor to the X-15, and a system to
replenish the X-15 LOX supply. The aircraft,
now designated 77 NB-52A, was flown to
Edwards AFB on 14 November 1958; it was
later named "The High and the Mighty One."
The Air Force also supplied a B-52B (52-008)
that arrived in Palmdale for similar modifica-
tions on 5 January 1959, and was flown, as an
NB-52B, to Edwards on 8 June 1959.
Roll Out
As the first X-15 was being completed, the
NACA held the second X-15 industry con-
ference in Los Angeles on 28-29 July 1958.
North American began the conference with a
paper detailing the developmental status of
the aircraft. Twenty-seven other papers cov-
ered subjects such as stability and control,
simulator testing, pilot considerations, mis-
sion instrumentation, thermodynamics,
structures, materials and fabrication. There
were approximately 550 attendees. 78
On 1 October 1958, High-Speed Flight
Station employees Doll Matay and John
Hedgepeth put up a ladder in front of the sta-
tion building at the foot of Lilly Avenue and
took down the winged-shield NACA emblem
from over the entrance door. NASA had
arrived in the desert, bringing with it a new era
of space-consciousness, soaring budgets, and
publicity. The old NACA days of concentra-
tion on aeronautics, and especially aerody-
namics, were gone forever, as was the agency
itself. On this day, the National Aeronautics
and Space Administration was created. 79
The X-15 construction process eventually
consumed just over two years, and on 15
October 1958, the first aircraft (56-6670)
was rolled out. Following conclusion of the
official ceremonies, it was moved back
inside and prepared for . shipment to
Edwards. On the night of 16 October, cov-
ered completely in protective heavy-duty
wrapping paper, it was shipped by truck to
Edwards for initial ground test work.
42
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 2
X-15 Design and Development
Chapter 2
Notes and
References
Letter from Scott Crossfield to Dennis R. Jenkins, 30 June 1999.
Harrison A. Storms, "X-15 Hardware Design Challenges" (a paper in the Proceedings of the X-15 30th Anniversary
Celebration, Dryden Flight Research Facility, Edwards, California, 8 June 1989, NASA CP-3105), p. 27.
Letter from Scott Crossfield to Dermis R. Jenkins, 30 June 1999.
The High-Speed Flight Station (HSFS) became the Flight Research Center (FRC) on 27 September 1959, and the
Hugh L. Dryden Flight Research Center (usually abbreviated DFRC) on 26 March 1976.
Wendell H. Stillwell, X-15 Research Results, (NASA, Washington, DC.: NASA SP-60, 1965).
James E. Love, "History and Development of the X-15 Research Aircraft," not dated, DFRC History Office, p. 10.
Eventually, an Air Force-NACA study team journeyed to France to study the prototype Sud-Ouest Trident intercep-
tor, which had such a tail configuration. See "Beyond the Frontiers, Sub-Quest Trident: Mixed-Powerplant Fighter,"
Wings nf Fame, Aerospace Publishing Ltd., London, Volume 10, p. 32.
Memorandum from M. A. Todd, Acting Chief, Contractor Reporting and Bailment Branch, Support Division, to Chief,
Fighter Branch, Aircraft Division, Director Procurement and Production, AMC, 15 June 1956, subject: Confirmation
of Serial Numbers Assigned, in the files of the AFMC History Office, Wright-Patterson AFB (WPAFB), Ohio.
Letter from Dr. Hugh L. Dryden, Director of NACA, to Chief, Fighter WSPO, Director of Systems Management,
ARDC, 6 July 1956, no subject, in the files of the AFMC History Office, WPAFB, Ohio.
Research Airplane Committee Report on the Conference on the Progress of the X-15 Project, a compilation of the
papers presented at the Langley Aeronautical Laboratory, Langley Field, Virginia, 25-26 October 1956.
Ibid., pp. 23-31.
The list included facilities at Langley, Ames, North American, and the Massachusetts Institute of Technology.
Dale L. Compton, "Welcome," (a paper in the Proceedings of the X-15 30th Anniversary Celebration, Dryden Flight
Research Facility, Edwards, California, 8 June 1989, NASA CP-3105), p. 3.
The interim XLR1 1 engine would use liquid oxygen as the oxidizer and an ethyl alcohol-water mixture as fuel.
S. A. Sjoberg, "Some Experience With Side Controllers" (a paper presented at the NACA Conference on the Progress
of the X-15 Project, Langley Aeronautical Laboratory, Langley Field, Virginia, 25-26 October 1956), pp. 167-171.
X-15 Interim Flight Manual, FHB-23-1, 18 March 1960, changed 12 May 1961.
Today this would be called a reaction control system.
Also known as the Q-ball.
A gimbal allows a body to incline in predefined directions. In this case the sphere could move both left-right and up-
down in relation to the nose. Electric servomotors (servos) provided the power to move the sphere as necessary.
Memorandum from Arthur W. Vogeley, Aeronautical Research Scientist, NACA, to Research Airplane Project Leader,
Langley Aeronautical Laboratory, NACA, 30 November 1955, subject: Project 1226 meetings to discuss changes in
the North American Proposal— Wright-Patterson AFB meeting of 24-25 October, and North American Aviation meet-
ings in Los Angeles on 27-28 October and 14-15 November 1955.
Walter C. Williams, "X-15 Concept Evolution" (a paper in the Proceedings of the X-15 30th Anniversary Celebration,
Dryden Flight Research Facility, Edwards, California, 8 June 1989, NASA CP-3105), p. 13.
Lawrence P. Greene, "Summary of Pertinent Problems and Current Status of the X-15 Airplane" (a paper presented
at the NACA Conference on the Progress of the X-15 Project, Langley Aeronautical Laboratory, Langley Field,
Virginia, 25-26 October 1956), pp. 239-258.
A "request for alteration" is the form used to request changes as the result of a mockup inspection within the Air Force.
Kinchloe had been selected as the Air Force project pilot for the X-15 program in September 1957. Unfortunately, he
was killed while attempting to eject from an F-104 at Edwards on 26 July 1958.
] Memorandum from T. J. Keating, Chief, Non-Rotating Engine Branch, Power Plant Laboratory, to Chief, New
Development Office, 15 November 1954, in the files of the AFMC History Office, WPAFB, Ohio.
1 In fairness to the laboratory, it must be admitted that such estimates were accompanied by a statement that "... less
confidence in these estimates exists because the XLR30 engine is at present in a much earlier stage of development."
This qualification proved to have been justified.
1 Memorandum from T. J. Keating, Chief, Non-Rotating Engine Branch, Power Plant Laboratory, to Chief, New
Development Office, 15 November 1954, in the files of the AFMC History Office, WPAFB, Ohio.
' Letter from Lieutenant H. J. Savage, Power Plant Laboratory, WADC, to Reaction Motors, Inc., 26 October 1955, sub-
ject: Engine for the X-15 Airplane, in the files of the AFMC History Office, WPAFB, Ohio.
' Letter from Lieutenant C. E. McCollough Jr., New Development Office, Director of WSO, WADC to Chief, Non-
Rotating Engine Branch, 1 December 1955, in the files of the AFMC History Office, WPAFB, Ohio.
' Letter from Rear Adm. W. A. Schoech, Assistant Chief for Research and Development, BuAer, USN, to C/S, USAF,
28 November 1955, subject: Cognizance over development of rocket power plant for NACA X-15 research airplane,
in the files of the AFMC History Office, WPAFB, Ohio.
' Letter from Brigadier General V. R. Haugen, Deputy Commander for Development, WADC, to NACA-Washington,
15 February 1956, subject: Engine Contract for the X-15, in the files of the AFMC History Office, WPAFB, Ohio.
1 Letter from W. P. Turner, Manager, Contracts Division, Reaction Motors, Inc., to Commander, AMC, 7 February 1956,
subject: Rocket Engine System for X-15 Research Aircraft., in the files of the AFMC History Office, WPAFB, Ohio
! Letter from W. P. Turner, Manager, Contracts Division, Reaction Motors, Inc., to Commander, AMC, 7 February 1956,
subject: Rocket Engine System for X-15 Research Aircraft, in the files of the AFMC History Office, WPAFB, Ohio.
' The designation became "official" at Wright-Field on 6 March and received Navy approval on 29 March. Many doc-
uments, particularly later in the flight program, list the designation as YLR99, but no evidence was discovered during
research for this monograph that indicated this was ever an official designation.
' Today liquid oxygen is usually abbreviated L02, but it was common practice up until the mod- 1980s to use LOX.
6 The engine was eventually to undergo numerous changes of detail but its basic design, as described to the conference,
was not greatly altered.
Monographs in Aerospace History Number 18 — Hypersonks Before the Shuttle
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X'l5 Design and Development Chapter 2
37 The Aircraft Engine Research Laboratory was founded on 23 June 1941;inApril 1947 it was renamed the Hight Propulsion
Research Laboratory. A year later it was renamed the Lewis Hight Propulsion Laboratory. When NASA was formed on 1
October 1958, the laboratory was renamed the Lewis Research Center (abbreviated LeRC to differentiate it from the Langley
Research Center — LaRQ. On 1 March 1999 it was renamed the John H. Glenn Research Center at Lewis Field.
38 Walter C. Williams, "X-l 5 Concept Evolution" (a paper in the Proceedings oftheX-15 30th Anniversary Celebration,
Dryden Flight Research Facility, Edwards, California, 8 June 1989, NASA CP-3105), p. 15.
39 Memorandum from Arthur W. Vogeley to Research Airplane Project Leader, NACA, 3 August 1957, subject: X-15
Airplane — Discussions at Air Research and Development Command, Detachment #1, Wright-Patterson Air Force
Base, Dayton, Ohio, on 29-30 July 1957, in the files of the AFMC History Office, WPAFB, Ohio.
40 Status Report of the XLR99-RM-1, 9 January 1958 through 27 June 1958, Prepared by Propulsion Laboratory,
WADC, in the files of the AFMC History Office, WPAFB, Ohio.
41 There was a clear distinction between proposals for an interim engine to permit flight trials before an XLR99 became
available, and an alternate engine, to substitute for the XLR99 in the final X-15.
42 Interview of C. E. McCollough, 14 May 1959, by R. S. Houston, in the files of the Air Force Museum archives.
43 The Power Plant and Propeller laboratories had been combined on 17 June 1957 into the Propulsion Laboratory.
44 X-15 WSPO Weekly Activity Report, 14 November 1958, in the files of the AFMC History Office, WPAFB, Ohio.
45 X-15 WSPO Weekly Activity Report, 24 April 1959, in the files of the AFMC History Office, WPAFB, Ohio.
46 X-15 WSPO Weekly Activity Report, 8 May 1959, in the files of the AFMC History Office, WPAFB, Ohio.
47 The flight engines were s/n 101 and 102; s/n 6 and 12 were the dedicated ground engines.
48 James E. Love, "History and Development of the X-15 Research Aircraft," not dated, DFRC History Office, p. 18.
45 Charles H. Feltz, "Description of the X-15 Airplane, Performance, and Design Mission" (a paper presented at the
NACA Conference on the Progress of the X-15 Project, Langley Aeronautical Laboratory, 25-26 October 1956), pp. 28.
50 X-15 WSPO Weekly Activity Report, 2 May 1958, in the files of the AFMC History Office, WPAFB, Ohio.
51 X-15 WSPO Weekly Activity Report, 5 September 1958, in the files of the AFMC History Office, WPAFB, Ohio.
52 Research Airplane Committee, Report on Conference on the Progress of the X-15 Project, compilation of the papers
presented at the IAS Building, Los Angeles, California, 28-30 July 1958, in files of X-15 WSPO, pp. 1 17.
53 Report, Detail Specifications NA5-4047, 2 March 1956, in the files of the AFMC History Office, WPAFB, Ohio.
54 X-15 WSPO Weekly Activity Report, 28 June 1956, in the files of the AFMC History Office, WPAFB, Ohio.
55 X-15 WSPO Weekly Activity Report, 30 January 1959; in the files of the AFMC History Office, WPAFB, Ohio.
56 James E. Love, "History and Development of the X-15 Research Aircraft," not dated, DFRC History Office, p. 13.
57 Memorandum from H. E. Savely, Chief, Biophysics Branch, Aero Medical Laboratory, WADC, to Chief, New
Development Office, Fighter Aircraft Division, Director of WSO, 8 February 1955, subject: Acceleration Tolerance
and Emergency Escape, in the files of the AFMC History Office, WPAFB, Ohio.
58 Memorandum from Arthur W. Vogeley, Aeronautical Research Scientist, NACA, to Research Airplane Project Leader,
30 November 1955, in the files of the AFMC History Office, WPAFB, Ohio.
59 Memorandum from Hartley Soule' to Members, NACA Research Airplane Project Panel, 7 June 1956, in the files of
the NASA History Office.
60 Memorandum from Brigadier General M. C. Dernier, Deputy Commander for R&D, to Deputy Commander for
Weapons Systems, ARDC, 2 January 1957, subject: Escape Systems for Research Vehicles such as the X-15, in the
files of the AFMC History Office, WPAFB, Ohio.
61 Letter from R. H. Rice, Vice President and General Manager, North American Aviation, to Commander, AMC, 31
January 1957, subject: Contract AF33(600)-31693, X-15 Airplane, GFAE Ejection Seat Catapult— Change to CFE
Ballistic Rocket Type-ECP NA-X-15-19, in the files of the AFMC History Office, WPAFB, Ohio.
62 X-15 WSPO Weekly Activity Report, 21 May 1958, in the files of the AFMC History Office, WPAFB, Ohio.
63 X-15 WSPO Weekly Activity Report, 13 March 1959, in the files of the AFMC History Office, WPAFB, Ohio.
64 Today, this is more often referred to as an inertial measurement unit, similar to what forms the basis of most inertial
navigation systems.
65 Memorandum from Walter C. Williams to Research Airplane Project Leader, 27 January 1956, in the files of the
AFMC History Office, WPAFB, Ohio.
66 Proposal Number A. E. 1752, "Development of Flight Research Stabilized Platform," Sperry Gyroscope Co.(contract
AF33-600-35397), in the files of the AFMC History Office, WPAFB, Ohio.
67 Contract AF33(600)-35397, 5 June 1957, in the files of the AFMC History Office, WPAFB, Ohio.
68 X-15 WSPO Weekly Activity Report, 2 May 1958, in the files of the AFMC History Office, WPAFB, Ohio.
69 X-15 WSPO Weekly Activity Report, 23 January 1959, in the files of the AFMC History Office, WPAFB, Ohio.
70 X-15 WSPO Weekly Activity Report, 13 March 1959, in the files of the AFMC History Office, WPAFB, Ohio.
71 X-15 WSPO Weekly Activity Report, 1 May 1959, in the files of the AFMC History Office, WPAFB, Ohio.
72 Interview, Lieutenant R. L. Panton, X-15 WSPO Director of Systems Management, ARDC, 1 June 1959, by R. S.
Houston, History Branch, WADC, in the files of the Air Force Museum archives.
73 James E. Love, "History and Development of the X-15 Research Aircraft," not dated, DFRC History Office, p. 20.
74 Walter C. Williams, "X-15 Concept Evolution" (a paper in the Proceedings of the X-15 30th Anniversary Celebration,
Dryden Flight Research Facility, Edwards, California, 8 June 1989, NASA CP-3105), p. 14.
75 Lawrence P. Greene, "Summary of Pertinent Problems and Current Status of the X-15 Airplane" (a paper presented at the
NACA Conference on the Progress of the X-15 Project, Langley Aeronautical Laboratory, 25-26 October 1956), p. 250.
76 Harrison A. Storms, "X-15 Hardware Design Challenges" (a paper in the Proceedings of the X-15 30th Anniversary
Celebration, Dryden Flight Research Facility, Edwards, California, 8 June 1989, NASA CP-3105), p. 27.
77 The "N" designation indicated that the aircraft had undergone permanent modifications to a non-standard configuration.
78 James E. Love, "History and Development of the X-15 Research Aircraft," not dated, DFRC History Office, p. 1 1.
79 Richard P. Haffion, On the Frontier: Flight Research at Dryden, (Washington, DC: NASA SP-4303, 1984); p. 101.
44 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 3
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Chapter 3
The Flight Research Program
Two different mission
profiles were flown —
one for maximum
speed; and one for
maximum altitude.
(NASA)
During the ten years of flight operations, five
major aircraft were involved in the X- 15 flight
research program. The three X-15s were des-
ignated X-15-1 (56-6670), X-15-2 (56-6671),
and X-15-3 (56-6672). Early in the test pro-
gram the first two X-15s were essentially iden-
tical in configuration; the third aircraft was
completed with different electronic and flight
control systems. When the second aircraft was
extensively modified after an accident mid-
way through the test program, it became the
X-15A-2. The two carrier aircraft were an
NB-52A (52-003) and an NB-52B (52-008);
they were essentially interchangeable.
The program used a three-part designation
for each flight. The first number represented
the specific X-15; 1 was for X-15-1, etc. No
differentiation was made between the origi-
nal X-15-2 and the modified X-15A-2. The
second position was the flight number for
that specific X-15. This included free-flights
only, not captive-carries or aborts; the first
flight was 1, the second 2, etc. If the flight
was a scheduled captive-carry, the second
position in the designation was replaced with
a C; if it was an aborted free-flight attempt,
it was replaced with an A. The third position
was the total number of times any X-15 had
been carried aloft by either NB-52. This
number incremented for each captive-carry,
abort, and actual release. The 24 May 1960
letter from FRC Director Paul Bikle estab-
lishing this system is included as an appen-
dix to this monograph.
Initial Flight Tests
The X-15-1 arrived at the Air Force Flight
Test Center at Edwards AFB, California, on
17 October 1958; trucked over the hills from
the North American plant in Los Angeles for
testing at the NASA- High-Speed Flight
Station. It was joined by the second airplane
in April 1959; the third would arrive later. In
contrast to the relative secrecy that had
attended flight tests with the XS-1 (X-l) a
decade before, the X-15 program offered the
spectacle of pure theater. 1
As part of the X-15's contractor program,
North American had to demonstrate each air-
craft's general airworthiness during flights
above Mach 2 before delivering it to the Air
Force, which would then turn it over to
NASA. Anything beyond Mach 3 was con-
sidered a part of the government's research
obligation. The contractor program would
last approximately two years, from mid-
1959 through mid-1960.
SPEED MISSION
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
45
The Flight Research Program
Chapter 3
The first X-1 5
(56-6670) immediately
prior to the official roll-
out ceremonies at
North American's Los
Angeles plant on 15
October 1958. The
small size of the
trapezoid-shaped
wings and the
extreme wedge sec-
tion of the vertical sta-
bilizer are noteworthy.
(North American
Aviation)
46
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 3
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Long before the
NB-52 first carried
the X-15 into the air,
engineers had tested
the separation charac-
teristics in the wind
tunnels at Langley
and Ames. Here an
X-15 model drops-
away from a model of
the NB-52. Note that
the X-15 is mounted
on the wrong wing.
This was necessary
because the viewing
area of the wind tun-
nel was on the left
side of the aircraft.
(NASA photo
EL-1 996-001 14)
The task of flying the X-15 during the con-
tractor program rested in the capable hands
of Scott Crossfield. After various ground
checks, the X-15-1 was mated to the
NB-52A, then more ground tests were con-
ducted. On 10 March 1959, the pair made a
scheduled captive-carry flight (program
fright number 1-C-l). They had a gross take-
off weight of 258,000 pounds, lifting off at
168 knots after a ground roll of 6,200 feet.
During the 1 hour and 8 minute flight it was
found that the NB-52 was an excellent carri-
er for the X-15, as had been expected from
numerous wind tunnel and simulator tests.
During the captive flight the X-15 flight con-
trols were exercised and airspeed data from
the flight test boom on the nose was obtained
in order to calibrate the instrumentation. The
penalties imposed by the X-15 on the NB-52
flight characteristics was found to be minimal
in the gear-up configuration. The mated pah-
was flown up to Mach 0.83 at 45,000 feet. 2
The next step was to release the X-15 from
the NB-52 in order to ascertain its gliding
and landing characteristics. The first glide
flight was scheduled for 1 April 1959, but
was aborted when the X-15 radio failed. The
pair spent 1 hour and 45 minutes airborne
conducting further tests in the mated config-
uration. A second attempt was aborted on 10
April 1959 by a combination of radio failure
and APU problems. Yet a third attempt was
aborted on 21 May 1959 when the X-15's
stability augmentation system failed, and a
bearing in the No. 1 APU overheated after
approximately 29 minutes of operation.
The problems with the APU were the most
disturbing. Various valve malfunctions,
leaks, and several APU speed-control prob-
lems were encountered during these three
flights, all of which would have been unac-
ceptable during research flights. Tests con-
ducted on the APU revealed that extremely
high surge pressures were occurring at the
pressure relief valve (actually a blow-out
plug) during initial peroxide tank pressuriza-
tion. The installation of an orifice in the heli-
um pressurization line immediately down-
stream of the shut-off valve reduced the
surges to acceptable levels. Other problems
were found to be unique to the captive-carry
flights and the long-run times being imposed
on the APUs; they were deemed to be of Ut-
ile consequence to the flight test program
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
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The Flight Research Program
Chapter 3
since the operating scenario would be differ-
ent. The APUs underwent a constant set of
minor improvements early in the flight test
program, but nevertheless continued to be a
source of irritation until the end.
On 22 May the first ground run of the inter-
im XLR11 engine installation was accom-
plished using the X-15-2. Scott Crossfield
was in the cockpit, and the test was consid-
ered successful, clearing the way for the
eventual first powered flight; if the first X-15
could ever make its scheduled glide flight.
Another attempt at the glide flight was made
on 5 June 1959 but was aborted even before
the NB-52 left the ground 3 when Crossfield
reported smoke in the X-15-1 cockpit.
Investigation showed that a cockpit ventila-
tion fan motor had overheated.
Finally, at 08:38 on 8 June 1959, Scott
Crossfield separated the X-15-1 from the
NB-52A at Mach 0.79 and 37,500 feet. Just
prior to launch the pitch damper failed, but
Crossfield elected to proceed with the flight,
and switched the SAS pitch channel to stand-
by. At launch, the X-15 separated cleanly
and Crossfield rolled to the right with a bank
angle of about 30 degrees. The X-15 touched
down on the dry lake at Edwards 4 minutes
and 56 seconds later. Just prior to landing,
the X-15 began a series of increasingly wild
pitching motions; mostly as a result of
Crossfield's instinctive corrective action, the
airplane landed safely. Landing speed was
150 knots, and the X-15 rolled-out 3,900 feet
while turning very slightly to the right. North
American subsequently modified the control
system boost to increase the control rate
response, effectively solving the problem.
Although the impact at landing was not con-
sidered to be particularly hard, later inspec-
tion revealed that bell cranks in both main
landing skids had bent slightly. The main
skids were not instrumented on this flight, so
no specific impact data could be ascertained,
but it was generally believed that the shock
struts had bottomed and remained bottomed
as a result of higher than predicted landing
loads. As a precaution against the main skid
problem occurring again, the metering char-
acteristics of the shock struts were improved,
and lakebed drop tests at higher than previ-
ous loads were made with the landing gear
test trailer that had been used to qualify the
landing gear design. All other airplane sys-
North American test
pilot A. Scott
Crossfield was
responsible for
demonstrating that the
X-1 5 was airworthy.
His decision to leave
NACA and join North
American effectively
locked him out of the
high-speed and high-
altitude test flights
later in the program.
(NASA photo
EC-570-1
48
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 3
The Flight Research Program
Any landing you can
walk away from ...
TheX-15-2madea
hard landing on 5
November 1959,
breaking its back as
the nose settled on
the lakebed. The dam-
age looked worse
than it was, and the
aircraft was back in
the air three months
later. (NASA photo
E-9543)
tems operated satisfactorily, clearing the way
for the first powered flight. 4
In preparation for the first powered flight,
the X-15-2 was taken for a captive-carry
flight with full propellant tanks on 24 July
1959. During August and early September,
several attempts to make the first powered
flight were cancelled before leaving the
ground due to leaks in the APU peroxide
system and hydraulic leaks. There were also
several failures of propellant tank pressure
regulators. Engineers and technicians
worked to eliminate these problems, all of
which were irritating, but none of which was
considered critical.
The first powered flight was made by X-15-2
on 17 September 1959. The aircraft was
released from the NB-52A at 08:08 in the
morning while flying at Mach 0.80 and
37,600 feet. Crossfield piloted the X-15-2 to
Mach 2.11 and 52,341 feet during 224.3 sec-
onds of powered flight using the two XLR11
engines. He landed on the dry lakebed at
Edwards 9 minutes and 11 seconds after
launch. Following the landing, a fire was
noticed in the area around the ventral stabiliz-
er, and was quickly extinguished by ground
crews. A subsequent investigation revealed
that the upper XLR1 1 fuel pump diffuser case
had cracked after engine shutdown and had
sprayed fuel throughout the engine compart-
ment. Fuel collected in the ventral stabilizer
and was ignited by unknown causes during
landing. No appreciable damage was done,
and the aircraft was quickly repaired. 5
The third flight of X-15-2 took place on 5
November 1959 when the X-15 was dropped
from the NB-52A at Mach 0.82 and 44,000
feet. During the engine start sequence, one
chamber in the lower engine exploded. There
was external damage around the engine and
base plate, plus quite a bit of damage internal
to the engine compartment. The resulting fire
convinced Crossfield to make an emergency
landing at Rosamond Dry Lake; he quickly
shut off the engines, dumped the remaining
fuel, and jettisoned the ventral 6 rudder. Even
so, within the 13.9 seconds of powered
flight, the X-15 managed to accelerate to
Mach 1. The aircraft touched down near the
center of the lake at approximately 150 knots
and an 11 degrees angle of attack. When the
nose gear bottomed out, the fuselage literal-
ly broke in half at station 7 226.8, with about
70 percent of the bolts at the manufacturing
Monographs in Aerospace History Number 18 — Hypersonics Be/ore the Shuttle
49
The Flight Research Program
Chapter 3
joint being sheared out. The fuselage contact-
ed the ground and was dragged for approxi-
mately 1,500 feet. Crossfield later stated that
the damage was the result of a defect that
should have broken on the first flight. 8 The
aircraft was sent to the North American plant
for repairs, and was subsequently returned to
Edwards in time for its fourth flight on 11
February I960. 9
The X-15-1 made its first powered flight,
using two XLRlls, on 23 January 1960.
This was also the first flight using the stable
platform, and the performance of the system
was considered encouraging. Under the
terms of the contract, the X-15 had still
"belonged" to North American until they had
demonstrated its basic airworthiness and
operation. Following this flight, a pre-deliv-
ery inspection was accomplished, and on 3
February 1960 the airplane was formally
accepted by the Air Force and subsequently
turned over to NASA.
The first government X-15 flight (1-3-8) was
on 25 March 1960 with NASA test pilot
Joseph A. Walker at the controls. The X-15-1
was launched at Mach 0.82 and 45,500 feet,
although the stable platform had malfunc-
tioned just prior to release. Two restarts were
required on the top engine before all eight
chambers were firing, and the flight lasted
just over 9 minutes, reaching Mach 2.0 and
48,630 feet. For the next six months, Walker
and Major Robert M. White alternated flying
the X-15-1. 10
It is interesting to note that the predictions
regarding flutter made by Lawrence P. Greene
at the first industry conference in 1956 did
materialize, although fortunately they were
not major and relatively easy to correct.
During the early test flights, vibrations at 1 10
cycles had been noted and were the cause of
some concern. Engineers at FRC added
instrumentation to the X-15s from flight to
flight in an attempt to isolate the vibrations
and understand their origins, while wind tun-
nel tests were conducted at Langley. It was
finally determined that the vibrations were
being caused by a flutter of the fuselage side
tunnel panels. These had been constructed in
removable sections with an unsupported
length of over 6 feet in some cases. 11 North
American added longitudinal stiffeners along
the underside of each panel, and this largely
cured the problem. 12
The X-15-1 flew three times in the two weeks
between 4 August and 19 August 1960, with
five aborted launches due to various problems
(including persistent APU failures). Two of
these flights were made by Joe Walker, and
one by Bob White. The flight on 12 August
was to an altitude of 136,500 feet, marking the
highest flight of an XLR1 1-powered X-15.
The Million Horsepower Engine 13
The X-15-3 had arrived at Edwards on 29
June 1959 but had not yet flown when the first
XLR99 flight engine (s/n 105) was installed
in it during early 1960. It should be noted that
the third X-15 was never equipped for the
XLR1 1 engines. At the same time, the second
X-15 was removed from flight status after its
ninth flight (2-9-18) on 26 April 1960, in
anticipation of replacing the XLR11 engines
with the new XLR99. This left only the
X-15-1 on active flight status.
The first ground run with the XLR99 in the
X-15-3 was made on 2 June 1960. Inspection
of the aircraft afterward revealed damage to
the liquid oxygen inlet fine brackets, the
result of a water-hammer effect. After repairs
were completed, another ground run was
conducted on 8 June. A normal engine start
and a short run at minimal power was made,
followed by a normal shutdown. A restart
was attempted, but was shutdown automati-
cally by a malfunction indication. Almost
immediately, a second restart was attempted,
resulting in an explosion that effectively
destroyed the aircraft aft of the wing.
Crossfield was in the cockpit, which was
thrown 30 feet forward, but he was not
injured. Subsequent investigation revealed
that the ammonia tank pressure regulator had
failed open. Because of some ground han-
50
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 3
The Flight Research Program
The top and bottom of
the fuselage were
usually covered in
frost because the LOX
tank was integral with
the fuselage. Oxygen
is liquid at -297
degrees Fahrenheit.
All three X-1 5s nor-
mally carried a yellow
NASA banner on their
vertical stabilizers.
(U.S. Air Force)
dling hoses attached to the fuel vent line, the
fuel pressure-relief valve did not operate
properly, thus allowing the fuel tank to over-
pressurize and rupture. In the process, the
peroxide tank was damaged by debris, and
the mixing of the peroxide and ammonia
caused an explosion.
Post-accident analysis indicated that there
were no serious design flaws with either the
XLR99 or the X-15. The accident had been
caused by a simple failure of the pressure reg-
ulator, exasperated by the unique configura-
tion required for the ground test. Modification
of the X-1 5-2 to accept the XLR99 continued,
and several other modifications were incorpo-
rated at the same time. These included a
revised vent system in the fuel tanks as an
additional precaution against another explo-
sion; revised ballistic control system compo-
nents; and provisions for the installation of the
ball-nose instead of the flight test boom that
had been used so far in the program. The
remains of the X-1 5-3 were returned to North
American, which received authorization to
rebuild the aircraft in early August. 14
The installation of the ball-nose presented its
own challenges since it had no capability to
determine airspeed. The X-15 was designed
with an alternate airspeed probe just forward
of the cockpit, although two other locations,
one well forward on the bottom centerline of
the aircraft, and one somewhat aft near the
centerline, had been considered alternate
locations. Several early flights compared the
data available from each location, while rely-
ing on the data provided by the airspeed sen-
sors on the flight test boom protruding from
the extreme nose. This indicated that the data
from all three locations were acceptable, so
the original location was retained. After the
ball-nose was installed, angle-of-attack data
was compared to that from previous flights
using the flight test boom; the data were gen-
erally in good agreement, clearing the way
for operational use of the ball-nose.
The first flight attempt of X-1 5-2 with the
XLR99 was made on 13 October 1960, but
was terminated prior to launch because of a
peroxide leak in the No. 2 APU. Just to show
haw many things could go wrong on a single
flight, there was also propellant impingement
on the aft fuselage during the prime cycle,
manifold pressure fluctuations during engine
turbopump operation, and fuel tank pressure
fluctuations during the jettison cycle.
Nevertheless, two weeks later, Crossfield
again entered the cockpit with the goal of
making the first XLR99 flight. Again, prob-
lems with the No. 2 APU forced an abort.
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
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The Flight Research Program
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On 15 November 1960, everything went right,
and Crossfield made the first flight of X-15-2
powered by the XLR99. The primary flight
objective was to demonstrate engine operation
at 50 percent thrust. The launch was at 46,000
feet and Mach 0.83, and even with only half
the available power, the X-15 managed to
climb to 81,200 feet and Mach 2.97. The sec-
ond XLR99 flight tested the engine's restart
and throttling capability. Crossfield made the
flight on 22 November, again using the sec-
ond X-15. The third and final XLR99
demonstration flight was accomplished using
X-15-2 on 6 December 1960. The objectives
of engine throttling, shutdown, and restart
were successfully accomplished. This marked
North American Aviation's, and Scott
Crossfield's, last X-15 flight. The job of fly-
ing the X-15 was now totally in the hands of
the government test pilots. 15
After this flight, a work schedule was estab-
lished which would permit an early flight
with a government pilot using North
American maintenance personnel. The flight
was tentatively scheduled for 21 December
1960 with Bob White as the pilot. However,
a considerable amount of work had to be
accomplished before the flight, including the
removal and replacement of the engine (s/n
103) which had suffered excessive chamber
coating loss, installation of redesigned
canopy hooks, installation of an unrestricted
upper vertical stabilizer, rearrangement of
the alternate airspeed system, and the reloca-
tion of the ammonia tank helium pressure
regulator into the fixed portion of the upper
vertical. During a preflight ground run, a
pinhole leak was found in the chamber throat
of the engine. Although the leak was found
to be acceptable for an engine run, it became
increasingly worse during the test until it
was such that the engine could not be run
again. Since there was no spare engine avail-
able, the flight was cancelled and a schedule
established to deliver the aircraft to the gov-
ernment prior to another flight. The X-15-2
was formally delivered to the Air Force and
turned over to NASA on 8 February 1961.
On the same day, X-15-1 was returned to the
North American plant for conversion to the
XLR99, having completed the last XLR11
flight of the program the day before with
White at the controls. 16
From the beginning of the X-15 flight test
program in 1959 until the end of 1960, a total
of 3 1 flights had been made with the first two
Six of the X-15 pilots
(from left to right):
Lieutenant Colonel
Robert A. Rushworth
(USAF), John B.
"Jack" McKay (NASA),
Lieutenant
Commander Forrest
S. Petersen (USN),
Joseph A. Walker
(NASA), Neil A
Armstrong (NASA),
Major Robert M. White
(USAF). (NASA via
the San Diego
Aerospace Museum
Collection)
52
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 3
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X-15s by seven pilots. But the X-15-1 was
experiencing an odd problem. When the
APU was started, hydraulic pressure was
either slow in coming up, or dropped off out
of limits when the control surfaces were
moved. The solution to the problem came
after additional instrumentation was placed
on the hydraulic system. The boot-strap line
which pressurized the hydraulic reservoir
was freezing, causing a flow restriction or
stoppage. Under these conditions, the
hydraulic pump would cavitate, resulting in
little or no pressure rise. The apparent cause
of this problem was the addition of a liquid
nitrogen line to cool the stable platform.
Since the nitrogen line was installed adjacent
to the hydraulic lines, it caused the Orinite
hydraulic oil to freeze. The solution to the
problem was to add electric heaters to the
affected hydraulic lines.
Joe Walker's flight on 30 March 1961
marked the first use of the new A/P-22S
full-pressure suit instead of the earlier MC-2.
Walker reported the suit was much more
comfortable and afforded better vision. But
the flight pointed out a potential problem
with the stability augmentation system
(SAS). As Walker descended through
100,000 feet, a heavy vibration occurred
and continued for about 45 seconds until
recovery was affected at 55,000 feet.
Incremental acceleration of approximately
1-g was noted in the vertical and transverse
axes at a frequency of 13 cycles. This cor-
responded to the first bending mode of the
horizontal stabilator. The center of gravity
of the horizontal surfaces was located
behind the hinge line; consequently rapid
surface movement produced both rolling
and pitching inertial moments. Subsequent
analysis showed the vibration was sustained
by the SAS at the natural frequency of the
horizontal surfaces. Essentially, the oscilla-
tions began because of the increased activi-
ty of the controls on reentry which excited
the oscillation and stopped after the pilot
reduced the pitch-damper gain. 17
Two solutions to the problem were discussed
between the FRC, North American, the Air
Force, and the manufacturer of the SAS,
Westinghouse; a notch filter for the SAS and
a pressure-derivative feedback valve for the
main stabilator hydraulic actuator. The notch
filter eliminated SAS control surface input at
13 cycles, and the feedback valve damped
the stabilator bending mode. In essence, the
valve corrected the source of the problem,
while the notch filter avoided the problem.
Although it was felt that either solution
would likely cure the problem, the final deci-
sion was to use both.
NASA research pilot William Dana made a
check flight in a specially-modified JF-100C
(53-1709) at Ames on 1 November I960,
delivering the aircraft to the FRC the follow-
ing day. The aircraft had been modified as a
variable-stability trainer that could simulate
the X-15's flight profile. This made it possi-
ble to investigate new piloting techniques
and control-law modifications without using
an X-15. Another 104 flights were made for
pilot checkout, variable stability research,
and X-15 support before the aircraft was
returned to Ames on 1 1 March 1964. 18
The first government flight with the XLR99
engine took place on 7 March 1961 with Bob
White at the controls. The X-15-2 reached
Mach 4.43 and 77,450 feet, and the flight was
generally satisfactory. The objectives of the
flight were to obtain additional aerodynamic
and structural heating data, as well as informa-
tion on stability and control of the aircraft at
high speeds. Post-flight examination showed a
limited amount of buckling to the side-fuse-
lage tunnels, attributed to thermal expansion.
The temperature difference between the tunnel
panels and the primary fuselage structure was
close to 500 degrees Fahrenheit. The damage
was not considered significant since the panels
were not primary structure, but were only nec-
essary to carry air loads. However, the buck-
ling continued to become more severe as
Mach numbers increased in later flights, and
eventually NASA elected to install additional
expansion joints in the tunnel skin to minimize
the buckling. 19
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
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The Flight Research Program
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By June 1961, government test pilots had
been, operating the X-15 on research flights
for just over a year. 20 The research phase of the
X-15's flight program involved four broad
objectives: verification of predicted hyperson-
ic aerodynamic behavior and heating rates,
study of the X-15's structural characteristics
in an environment of high heating and high
flight loads, investigation of hypersonic sta-
bility and control problems during atmospher-
ic exit and reentry, and investigation of pilot-
ing tasks and pilot performance. By late 1961,
these four areas had been generally examined,
although detailed research continued to about
1964 using the first and third aircraft, and to
1967 with the second (as the X-15A-2).
Before the end of 1961, the X-15 had attained
its Mach 6 design goal and had flown well
above 200,000 feet; by the end of 1962 the X-
15 was routinely flying above 300,000 feet.
The X-15 had already extended the range of
winged aircraft flight speeds from Mach 3.2 21
to Mach 6.04, the latter achieved by Bob
White on 9 November 1961.
The X-15 flight research program revealed a
number of interesting things. Physiologists
discovered the heart rates of X-15 pilots var-
ied between 145 and 185 beats per minute in
flight, as compared to a normal of 70 to 80
beats per minute for test missions in other
aircraft. Researchers eventually concluded
that pre-launch anticipatory stress, rather
than actual post launch physical stress, influ-
enced the heart rate. They believed, correct-
ly, that these rates could be considered as
probable baselines for predicting the physio-
logical behavior of future astronauts.
Aerodynamic researchers found remarkable
agreement between the wind tunnel tests of
exceedingly small X-15 models and actual
results, with the exception of drag measure-
ments. Drag produced by the blunt aft end of
the actual aircraft proved 15 percent higher
than wind tunnel tests had predicted.
At Mach 6, the X-15 absorbed eight times
the heating load it experienced at Mach 3,
with the highest heating rates occurring in
the frontal and lower surfaces of the aircraft,
which received the brunt of airflow impact.
During the first Mach 5+ flight, four expan-
sion slots in the leading edge of the wing
generated turbulent vortices that increased
heating rates to the point that the external
skin behind the joints buckled. It offered "...
a classical example of the interaction among
aerodynamic flow, thermodynamic proper-
ties of air, and elastic characteristics of struc-
ture." As a solution, small Inconel X alloy
strips were added over the slots and addi-
tional fasteners on the skin. 22
Heating and turbulent flow generated by the
protruding cockpit enclosure posed other
problems; on two occasions, the outer panels
of the X-15's glass windshields fractured
because heating loads in the expanding
frame overstressed the soda-lime glass. The
difficulty was overcome by changing the
cockpit frame from Inconel X to titanium,
eliminating the rear support (allowing the
windscreen to expand slightly), and replac-
ing the outer glass panels with high temper-
ature alumina silica glass. All this warned
aerospace designers to proceed cautiously.
During 1968 John Becker 23 wrote: "The real-
ly important lesson here is that what are
minor and unimportant features of a subson-
ic or supersonic aircraft must be dealt with as
prime design problems in a hypersonic air-
plane. This lesson was applied effectively in
the precise design of a host of important
details on the manned space vehicles."
A serious roll instability predicted for the
airplane under certain reentry conditions
posed a dilemma to flight researchers. To
accurately simulate the reentry profile of a
returning winged spacecraft, the X-15 had to
fly at angles of attack of at least 17 degrees.
Yet the wedge-shaped vertical and ventral
stabilizers, so necessary for stability and
control in other portions of the flight regime,
actually prevented the airplane from being
flown safely at angles of attack greater than
20 degrees because of potential rolling prob-
lems. By this time, FRC researchers had
gained enough experience with the XLR99
engine to realize that fears of thrust mis-
54
Hypersonics Before the Shuttle — Monographs in Aerospace History "Number 18
Chapter 3
The Flight Research Program
A common sight dur-
ing the 1 960s over
Edwards— an NB-52
carrying an X-1 5. This
was a boy's dream at
the time; and the sub-
ject of many fantasies.
Over the course of the
program, the markings
on the NB-52s
changed significantly.
Early on, they were
natural metal with
bright orange verti-
cals; later they were
overall gray. (NASA)
alignment — a major reason for the large sur-
faces — were unwarranted. The obvious solu-
tion was simply to remove the lower portion
of the ventral, something that X-1 5 pilots
had to jettison prior to landing anyway so
that the aircraft could touch down on its
landing skids. Removing part of the ventral
produced an acceptable tradeoff; while it
reduced stability by about 50 percent at high
angles of attack, it greatly improved the
pilot's ability to control the airplane. With
the ventral off, the X-1 5 could fly into the
previously "uncontrollable" region above 20
degrees angle of attack with complete safety.
Eventually the X-1 5 went on to reentry tra-
jectories of up to 26 degrees, often with
flight path angles of -38 degrees at speeds
up to Mach 6. M Its reentry characteristics
were remarkably similar to those of the later
Space Shuttle orbiter.
When Project Mercury began, it rapidly
eclipsed the X-15 in the public's imagina-
tion. It also dominated some of the research
areas that had first interested X-15 planners,
such as "zero-g" weightlessness studies. The
use of reaction controls to maintain attitude
in space proved academic after Mercury
flew, but the X-15 would furnish valuable
information on the blending of reaction con-
trols with conventional aerodynamic con-
trols during exit and reentry, a matter of con-
cern to subsequent Shuttle development. The
X-15 experience clearly demonstrated the
ability of pilots to fly rocket-propelled air-
craft out of the atmosphere and back in to
precision landings. Paul Bikle saw the X-15
and Mercury as a "... parallel, two-pronged
approach to solving some of the problems of
manned space flight. While Mercury was
demonstrating man's capability to function
effectively in space, the X-15 was demon-
strating man's ability to control a high per-
formance vehicle in a near-space environ-
ment ... considerable new knowledge was
obtained on the techniques and problems
associated with lifting reentry." 25
Nearly all of the early XLR99 flights experi-
enced malfunction shutdowns of the engine
immediately after launch, and sometimes
after normal engine shutdown or burnout.
Since the only active engine system after
shutdown was the lube-oil system, investiga-
tions centered on it. Analyses of this condi-
tion revealed very wide acceleration excur-
sions during the engine-start phase. A rea-
sonable simulation of this acceleration was
accomplished by placing an engine on a
work stand with the ability to rotate the
engine about the Y-axis. Under certain con-
ditions, the lube-oil pump could be made to
Monographs in Aerospace History Number 18 — H^personics Before the Shuttle
55
The Flight Research Program
Chapter 3
cavitate for about 2 seconds, tripping an
automatic malfunction shutdown. To elimi-
nate this problem, a delay timer was installed
in the lube-oil malfunction circuit which
allowed the pump to cavitate up to 6 seconds
without actuating the malfunction shutdown
system. After this delay timer was installed
in early 1962, no further engine shutdowns
of this type were experienced. 26
But a potentially more serious XLR99 prob-
lem was the unexpected loss of the Rokide
coating from the combustion chamber during
firing. A meeting was held at Wright Field on
13 June 1961 to discuss possible solutions. It
was decided that the Wright Field Materials
Laboratory would develop a new ceramic
coating for the chambers, and that FRC
would develop the technique and fixtures
required to recoat chambers at Edwards.
Originally, the Materials Laboratory award-
ed a contract to Plasmakote Corp. to perform
the coating of several chambers, but the
results were unsatisfactory. By March 1962,
the techniques and fixtures developed by the
FRC allowed chambers to be successfully
recoated at Edwards.
Early in the program, the X-15's stability
augmentation and inertial guidance systems
were two major problem areas. NASA even-
tually replaced the Sperry inertial unit with a
Honeywell system designed for the stillborn
Dyna-Soar. The propellant system had its
own weaknesses; pneumatic vent and relief
valves and pressure regulators gave the
greatest difficulties, followed by spring pres-
sure switches in the APUs, the turbopump,
and the gas generation system. NASA's
mechanics routinely had to reject 24-30 per-
cent of spare parts as unusable, a clear indi-
cation of the difficulties that would be expe-
rienced later in the space programs in getting
parts manufactured to exacting specifica-
tions. 27 Weather posed a critical factor. Many
times Edwards enjoyed good weather while
other locations on the High Range were cov-
ered with clouds, alternate landing sites were
flooded, or some other meteorological con-
dition postponed a mission.
Follow-on Experiments
During the summer of 1961, a new research
initiative was proposed by the Air Force's
Aeronautical Systems Division at Wright-
Patterson AFB and NASA Headquarters:
using the X- 15 to carry a wide range of sci-
On 4 November 1960,
the program attempt-
ed to launch twoX-15
flights in a single day.
Here X-1 5-1 is mount-
ed on the NB-52B
and X-1 5-2 is on the
NB-52A. Rushworth
was making his first
flight in X-1 5-1, a low
(48,900 feet) and slow
(Mach 1 .95) familiar-
ization. The X-1 5-2,
with Crossfield as
pilot, aborted due to a
failure in the No. 2
APU. (NASA photo
E-6186)
56
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 3
The Flight Research Program
entific experiments unforeseen when the air-
craft was conceived in 1954.
Researchers at the FRC wanted to use the X-15
to carry high-altitude experiments related to
the proposed Orbiting Astronomical
Observatory; others suggested modifying
one of the airplanes to carry a Mach 5+ ram-
jet for advanced air-breathing propulsion
studies. Over 40 experiments were suggested
by the scientific community as suitable can-
didates for the X-15 to carry. In August 1961
NASA and the Air Force formed the "X-15
Joint Program Coordinating Committee" to
prepare a plan for a follow-on experiments
program. The committee held its first meet-
ing on 23-25 August 1961 at the FRC. 28
Many experiments suggested to the commit-
tee related to space science, such as ultravio-
let stellar photography. Others supported the
Apollo program and hypersonic ramjet stud-
ies. Hartley Soule and John Stack, then
NASA's director of aeronautical research,
proposed the classification of experiments
into two groups: category A experiments
consisted of well-advanced and funded
experiments having great importance; cate-
gory B included worthwhile projects of less
urgency or importance. 29
In March 1962 the committee approved the
"X-15 Follow-on Program," and NASA
announced that an ultraviolet stellar photog-
raphy experiment from the University of
Wisconsin's Washburn Observatory would
be first. The X-15's space science program
eventually included twenty-eight experi-
ments including astronomy, micrometeorite
collection (using wing-top pods on the X-15-
1 and X-15-3 that opened at 150,000 feet),
and high-altitude mapping. The micromete-
orite experiment was unsuccessful, and was
ultimately cancelled. Two of the follow-on
programs, a horizon definition experiment
from the Massachusetts Institute of
Technology, and test of insulation material
for the Saturn launch vehicle, directly bene-
fited the Apollo program. The Saturn insula-
tion was applied to the X-15's speed brakes,
which were then deployed at the desired
speed and dynamic pressure to test both the
insulating properties and the bonding materi-
al. By the end of 1964, over 65 percent of
data being returned from the three X-15 air-
craft involved follow-on projects; this per-
centage increased yearly through conclusion
of the program. 30
As early as May 1962, North American had
proposed modifying one of the X-15s as a
flying test bed for hypersonic engines. Since
the X-15s were being fully utilized at the
time, neither the Air Force nor NASA
expressed much interest in pursuing the idea.
However, when the X-15-2 was damaged
during a landing accident on 9 November
1962 (seriously injuring Jack McKay, who
would later return from his injuries to fly the
X-15 again), North American proposed mod-
ifying the aircraft in conjunction with its
repairs. General support for the plan was
found within the Air Force, which was will-
ing to pay the estimated $6 million. 31
On the other hand, NASA was less enthusias-
tic, and felt the aircraft should simply be
repaired to its original configuration. 32
Researchers at NASA believed that the Mach
8 X-15 would prove to be of limited value for
propulsion research. However, NASA did not
press its views, and in March 1963 the Air
Force authorized North American to rebuild
the aircraft as the X-15A-2. Twenty-nine
inches were added to the fuselage between
the existing propellant tanks. The extra vol-
ume was to be used by a liquid hydrogen tank
to power the ramjet, but the LH2 tank could
be replaced by other equipment as needed. In
fact, the compartment was frequently used to
house cameras to test reconnaissance con-
cepts, or to observe the dummy ramjet during
flight tests, through three heat-resistant win-
. dows in the lower fuselage. The capability to
carry two external propellant tanks was
added to provide additional powered flight
with the XLR99. The right wingtip was also
modified to allow various wingtip shapes to
be carried interchangeably, although it
appears that this capability was never used. 33
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
57
The Flight Research Program
Chapter 3
Forty weeks and $9 million later, North
American delivered the X-15A-2. 34 The air-
craft made its first flight on 25 June 1964
piloted by Bob Rushworth. Early flights
demonstrated that the aircraft retained satis-
factory flying qualities at Mach 5, although
on three flights thermal stresses caused por-
tions of the landing gear to extend at Mach
4.3, generating "an awful bang and a yaw." 35
In each case Rushworth landed safely, despite
the blow-out of the heat-weakened tires in
one instance. On 18 November 1966, Pete
Knight set an unofficial world's speed record
of Mach 6.33 in the aircraft. The drop tanks
had been jettisoned at Mach 2.27 and 69,700
feet. A nonfunctional dummy ramjet was
constructed in order to gather aerodynamic
data on the basic shape in preparation for
possible flight tests in the early 1970s. The
first flight with the dummy ramjet attached to
the ventral was on 8 May 1967. Although
providing a pronounced nose-down trim
change, the ramjet actually restored some of
the directional stability lost when the lower
ventral rudder had been removed.
NASA had evaluated several possible coat-
ings that could be applied over the X-15's
Inconel X hot-structure to enable it to with-
stand the thermal loads experienced above
Mach 6. The use of such coatings could be
beneficial since various ablators were being
investigated by the major aerospace contrac-
tors during the early pre-concept phases 36 of
the Space Shuttle development. 37 Such a coat-
ing would have to be relatively light, have
good insulating properties, and be easy to
apply, remove, and reapply before another
flight. The selected coating was MA-25S, an
ablator developed by the Martin Company in
connection with some early reusable space-
craft studies. Consisting of a resin base, a cat-
alyst, and a glass bead powder, it would pro-
tect the hot-structure from the expected 2,000
degrees Fahrenheit heating at Mach 8. Martin
estimated that the coating, ranging from 0.59
inches thick on the canopy, wings, vertical,
and horizontal stabilizers, down to 0.015 inch-
es on the trailing edges of the wings and tail,
would keep the skin temperature below 600
degrees Fahrenheit. The first unpleasant sur-
prise came, however, with the application of
the coating to the X-15A-2: it took six weeks.
Getting the correct thickness over the entire
surface proved harder than expected. Also,
every time a panel had to be opened to service
the X-15, the coating had to be removed and
reapplied around the affected area.
Because the ablator would char and emit a
residue in flight, North American had
installed an "eyelid" over the left, cockpit
window; it would remain closed until just
before landing. During launch and climbout,
the pilot would use the right window, but
residue from the ablator would render it
opaque above Mach 6. The eyelid had
already been tested on several flights. 38
Late in the summer of 1967, the X-15A-2
was ready for flight with the ablative coat-
ing. The weight of the ablator — 125 pounds
higher than planned — together with expected
increased drag reduced the theoretical maxi-
mum performance of the airplane to Mach
7.4, still a significant advance over the Mach
6.3 previously attained. The appearance of
the X-15A-2 was striking, an overall flat off-
white finish, the external tanks a mix of sil-
ver and orange-red with broad striping. On
21 August 1967, Knight completed the first
flight in the ablative coated X-15A-2, reach-
ing Mach 4.94 and familiarizing himself
with its handling qualities. His next flight
was destined to be the program's fastest
flight, and the last flight of the X-15A-2. 39
On 3 October 1967, 43,750 feet over Mud
Lake, Knight dropped away from the
NB-52B. The flight plan showed the X-15A-2
would weigh 52,117 pounds at separation,
more than 50 percent heavier than originally
conceived in 1954. 40 The external tanks were
jettisoned 67.4 seconds after launch at Mach
2.4 and 72,300 feet; tank separation was satis-
factory, however, Knight felt the ejection was
"harder" than the last one he had experienced
(2-50-89). The recovery system performed
satisfactorily and the tanks were recovered in
repairable condition. The XLR99 burned for
58
Hypertonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 3
The Flight Research Program
140.7 seconds before Knight shut it down.
Radar data showed the X-15A-2 attained
Mach 6.70 (4,520 mph) at 102,700 feet, a
winged-vehicle speed record that would stand
until the return of the Space Shuttle Columbia
from its first orbital flight in 198 1. 41
The post-landing inspection revealed many
things. The ability of the ablative material to
protect the aircraft structure from the high
aerodynamic heating was considered good
except in the area around the dummy ramjet
where the heating rates were significantly
higher than predicted. The instrumentation
on the dummy ramjet had ceased working
approximately 25 seconds after engine shut-
down, indicating that a burn through of the
ramjet/pylon structure had occurred. Shortly
thereafter the heat propagated upward into
the lower aft fuselage causing the hydrogen-
peroxide hot light to illuminate in the cock-
pit. Assuming a genuine overheat condition,
William Dana in the NASA 1 control room
had requested Knight to jettison the remain-
ing peroxide. The high heat in the aft fuse-
lage area also caused a failure of a helium
check valve allowing not only the normal
helium source gas to escape, but also the
emergency jettison control gas supply as
well. Thus, the remaining residual propel-
lants could not be jettisoned. The aircraft
was an estimated 1,500 pounds heavier than
normal at landing, but the landing occurred
without incident.
wave impinged on the ramjet and its sup-
porting structure. The heat in the ramjet
pylon area was. later estimated to be ten times
normal, and became high enough at some
time during the flight to ignite 3 of the 4
explosive bolts holding the ramjet to the
pylon. As Knight was turning downwind in
the landing pattern, the one remaining bolt
failed structurally and the ramjet separated
from the aircraft. Knight did not feel the
ramjet separate, and since the chase aircraft
had not yet joined up, was unaware that the
ramjet had separated.
The position of the X- 15 at the time of sepa-
ration was later established by radar data and
the most likely trajectory estimated. A
ground search party discovered the ramjet on
the Edwards bombing range. Although it had
been damaged by impact, it was returned for
study of the heat damage.
The unprotected right-hand windshield was,
as anticipated, partially covered with ablation
products. Since the left eyelid remained
closed until well into the recovery maneuver,
Knight flew the X-15 using on-board instru-
ments and directions from William Dana in
the NASA 1 control room. The eyelid was
opened at approximately Mach 1.6 as the air-
craft was over Rogers Dry Lake, and the visi-
bility was considered satisfactory. Knight
landed at Edwards 8 minutes and 12 seconds
after launch.
An internal general
arrangement of the
modified X-15A-2.
(NASA)
Engineers had not fully considered possible
shock interaction with the ramjet shape at
hypersonic speeds. As it turned out, the flow
patterns were such that a tremendous shock
The ablator obviously was not totally success-
ful; in fact this was the closest any X-15 came
to structural failure induced by heating. Post-
flight inspection revealed that the aircraft was
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Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
59
The Flight Research Program
Chapter 3
charred on its leading edges and nose. The
ablator had actually prevented cooling of
some hot spots by keeping the heat away from
the hot-structure. Some heating effects, such
as where shock waves impinged on the ramjet
had not been thoroughly studied. To John
Becker the flight underscored ". . . the need for
maximum attention to aerothermodynamic
detail in design and preflight testing." 42 To
Jack Kolf, an X-15 project engineer at the
FRC, the post-flight condition of the airplane
". . . was a surprise to all of us. If there had
been any question that the airplane was going
to come back in that shape, we never would
have flown it." 43
Some of the problems encountered with the
ablator were nonrepresentative of possible
future uses. The X-15 had been designed as
an uninsulated hot structure. Any future
vehicle would probably be designed with a
more conventional airframe, eliminating
some of the problems encountered on this
flight. But some of the problems were very
real. The amount of time it took to apply the
ablator was unacceptable. Even considering
that the learning curve was steep, and that
after some experience the time could be cut
in half or even further, the six weeks it took
to coat the relatively small X-15 bode ill for
larger vehicles. Nevertheless, ablators would
continue to be proposed on various Space
Shuttle concepts, in decreasing quantity,
until 1970 when several forms of ceramic
tiles and metal "shingles" would become the
preferred concepts. 44
It was estimated that repairing the X-15A-2
and refurbishing the ablator for another flight
near Mach 7 would have taken five weeks.
The unexpected airflow problems around the
ramjet ended any idea of flying it again.
NASA sent the X-15A-2 to North American
for general maintenance and repair, and
although the aircraft returned to Edwards in
June 1968, it never flew again. It is now on
exhibit — in natural black finish — at the Air
Force Museum, Wright-Patterson AFB, Ohio.
The X-15A-2 drops
away from the NB-52
on its last flight. Note
the dummy ramjet
attached to the ventral
and the overall white
finish applied to the
ablator. The drop
tanks would be jetti-
soned 67.4 seconds
after engine ignition,
at a speed of Mach
2.4 and 72,300 feet
altitude. Pete Knight
would attain Mach
6.70 on this flight.
(NASA)
60
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 3
The Flight Research Program
Ultimately, Garrett did deliver a functioning
model of the ramjet, and it was successfully
tested in a wind tunnel in late 1969. In this
case successful meant that supersonic com-
bustion was achieved, although for a very
short duration and under very controlled and .
controversial conditions. 45
Adaptive Controls
The X-15-3 featured specialized flight
instrumentation and displays that rendered it
particularly suitable for high-altitude flight
research. A key element was the Minneapolis
Honeywell MH-96 "adaptive" flight control
system originally developed for the X-20
Dyna-Soar. This system automatically com-
pensated for the airplane's behavior in vari-
ous flight regimes, combining the aerody-
namic control surfaces and the reaction con-
trols into a single control package. This was
obviously the way future high-speed aircraft
and spacecraft would be controlled, but the
technology of the 1960s were severely taxed
by the requirements for such a system.
By the end of 1963, the X-15-3 had flown
above 50 miles altitude. This was the altitude
that the Air Force recognized as the mini-
mum boundary of space flight, and five Air
Force pilots were awarded Astronaut Wings
for their flights in the X-15. 46 All but one of
these flights was with X-15-3 (Astronaut Joe
Engle's first space flight was in X-15-1).
NASA did not recognize the 50 mile criteria,
using the international 62 mile standard
instead. Only a single NASA pilot went this
high; Joe Walker set a record for winged
space flight by reaching 354,200 feet (67
miles), a record that stood until the orbital
flight of Columbia nearly two decades later.
By mid-1967, the X-15-3 had completed
sixty-four research flights, twenty-one at
altitudes above 200,000 feet. It became the
primary aircraft for carrying experiments to
high altitude.
The X-15-3 would also make the most tragic
flight of the program. At 10:30 in the morning
on 15 November 1967, the X-15-3 dropped
away from the NB-52B at 45,000 feet over
Delamar Dry Lake. At the controls was Major
Michael J. Adams, making his seventh X-15
flight. Starting his climb under full power, he
was soon passing through 85,000 feet. Then
an electrical disturbance distracted him and
slightly degraded the control of the aircraft;
having adequate backup controls, Adams con-
tinued on. At 10:33 he reached a peak altitude
of 266,000 feet. In the NASA 1 control room,
mission controller Pete Knight monitored the
mission with a team of engineers. As the X-15
climbed, Adams started a planned wing-rock-
ing maneuver so an on-board camera could
scan the horizon. The wing rocking quickly
became excessive, by a factor of two or three.
At the conclusion of the wing-rocking portion
of the climb, the X-15 began a slow drift in
heading; 40 seconds later, when the aircraft
reached its maximum altitude, it was off head-
ing by 15 degrees. As Adams came over the
top, the drift briefly halted, with the airplane
yawed 15 degrees to the right. Then the drift
began again; within 30 seconds, Adams was
descending at right angles to the flight path.
At 230,000 feet, encountering rapidly increas-
ing dynamic pressures, the X-15 entered a
Mach 5 spin. 47
In the NASA 1 control room there was no
way to monitor heading, so nobody suspect-
ed the true situation that Adams now faced.
The controllers did not know that the air-
plane was yawing, eventually turning com-
pletely around. In fact, Knight advised
Adams that he was "a little bit high," but in
"real good shape." Just 15 seconds later,
Adams radioed that the aircraft "seems
squirrely." At 10:34 came a shattering call:
"I'm in a spin, Pete." Plagued by lack of
heading information, the control room staff
saw only large and very slow pitching and
rolling motions. One reaction was "disbelief;
the feeling that possibly he was overstating
the case." But Adams again called out, "I'm
in a spin." As best they could, the ground
controllers sought to get the X-15 straight-
ened out. There was no recommended spin
recovery technique for the X-15, and engi-
neers knew nothing about the aircraft's
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
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The Flight Research Program
Chapter 3
realizing that the X-15 would never make
Rogers Dry Lake, went into afterburner and
raced for the emergency lakes; Ballarat and
Cuddeback. Adams held the X-15's controls
against the spin, using both the aerodynamic
control surfaces and the reaction controls.
Through some combination of pilot tech-
nique and basic aerodynamic stability, the
airplane recovered from the spin at 118,000
feet and went into an inverted Mach 4.7 dive
at an angle between 40 and 45 degrees. 48
Adams was in a relatively high altitude dive
and had a good chance of rolling upright,
pulling out, and setting up a landing. But now
came a technical problem; the MH-96 began
a limit-cycle oscillation just as the airplane
came out of the spin, preventing the gain
changer from reducing pitch as dynamic
pressure increased. The X-15 began a rapid
pitching motion of increasing severity, still in
a dive at 160,000 feet per minute, dynamic
pressure increasing intolerably. As the X-15
neared 65,000 feet, it was diving at Mach
3.93 and experiencing over 15-g vertically,
both positive and negative, and 8-g laterally.
The aircraft broke up northeast of the town
of Johannesburg 10 minutes and 35 seconds
after launch. A chase pilot spotted dust on
Cuddeback, but it was not the X-15. Then an
Air Force pilot, who had been up on a
delayed chase mission and had tagged along
on the X-15 flight to see if he could fill in for
an errant chase plane, spotted the main
wreckage northwest of Cuddeback. Mike
Adams was dead; the X-15-3 destroyed. 49
NASA and the Air Force convened an acci-
dent board. Chaired by NASA's Donald R.
Bellman, the board took two months to pre-
pare its report. Ground parties scoured the
countryside looking for wreckage; critical to
the investigation was the film from the cock-
pit camera. The weekend after the accident,
an unofficial FRC search party found the
camera; disappointingly, the film cartridge
was nowhere in sight. Engineers theorized
that the film cassette, being lighter than the
camera, might be further away, blown north
by winds at altitude. FRC engineer Victor
Horton organized a search and on 29
November, during the first pass over the
area, Willard E. Dives found the cassette.
Most puzzling was Adams' complete lack of
awareness of major heading deviations in
spite of accurately functioning cockpit instru-
mentation. The accident board concluded that
he had allowed the aircraft to deviate as the
result of a combination of distraction, misin-
terpretation of his instrumentation display,
and possible vertigo. The electrical distur-
bance early in the flight degraded the overall
effectiveness of the aircraft's control system
and further added to pilot workload. The
MH-96 adaptive control system then caused
the airplane to break up during reentry. The
board made two major recommendations:
install a telemetered heading indicator in the
control room, visible to the flight controller;
and medically screen X-15 pilot candidates
for labyrinth (vertigo) sensitivity. 50 As a result
of the X-15's crash, the FRC added a ground-
based "8 ball" attitude indicator in the control
room to furnish mission controllers with real
time pitch, roll, heading, angle of attack, and
sideslip information.
Mike Adams was posthumously awarded
Astronaut Wings for his last flight in the
X-15-3, which had attained an altitude of
266,000 feet— 50.38 miles. In 1991 Adams'
name was added to the Astronaut Memorial
at the Kennedy Space Center in Florida.
The X-15 program would only fly another
eight missions. The X-15A-2, grounded for
repairs, soon remained grounded forever.
The X-15-1 continued flying, with sharp dif-
ferences of opinion about whether the
research results returned were worth the risk
and expense.
A proposed delta wing modification to the
X-15-3 had offered supporters the hope that
the program might continue to 1972 or 1973.
The delta wing X-15 had grown out of stud-
ies in the early 1960s on using the X-15
as a hypersonic cruise research vehicle.
62
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 3
The Flight Research Program
Essentially, the delta wing X-15 would have
made use of the third airframe with the adap-
tive flight control system, but also incorporat-
ed the modifications made to the X-15A-2 —
lengthening the fuselage, revising the land-
ing gear, adding external propellant tanks,
and provisions for a small-scale experimen-
tal ramjet. NASA proponents, particularly
John Becker at Langley, found the idea very
attractive since: "The highly swept delta
wing has emerged from studies of the past
decade as the form most likely to be utilized
on future hypersonic flight vehicles in which
high lift/drag ratio is a prime requirement
i.e., hypersonic transports and military
hypersonic cruise vehicles, and certain
recoverable boost vehicles as well." 51
Despite such endorsement, support remained
lukewarm at best both within NASA and the
Air Force; the loss of Mike Adams and the
X-15-3 effectively ended all thought of such
a modification.
As early as March 1964, in consultation with
NASA Headquarters, Brigadier General
James T. Stewart, director of science and tech-
nology for the Air Force, had determined to
end the X-15 program by 1968. 52 At a meeting
of the Aeronautics/Astronautics Coordinating
Board on 5 July 1966, it was decided that
NASA should assume total responsibility for
all X-15 costs (other than incidental AFFTC
support) on 1 January 1968. 53 This was later
postponed one year. As it turned out, by
December 1968 only the X-15-1 was still fly-
ing, and it cost roughly $600,000 per flight.
Other NASA programs could benefit from
this funding, and thus NASA did not request a
continuation of X-15 funding after December
1968. 54 During 1968 William Dana and Pete
Knight took turns flying the X-15-1. On 24
October 1968, Dana completed the X-15's
199th, and as it turned out the last, flight
reaching Mach 5.38 at 255,000 feet. A total of
ten attempts were made to launch the 200th
flight, but a variety of maintenance and
weather problems forced cancellation every
time. On 20 December 1968, the X-15-1 was
demated from the NB-52A for the last time.
After nearly a decade of flight operations, the
X-15 program came to an end.
The instrument panel
of the X-15-3 with the
MH-96 adaptive con-
trol system installed.
The dark panel imme-
diately ahead of the
center control stick
allowed the pilot to
control how the
MH-96 reacted.
(NASA photo
E63-9834) [
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
63
The Flight Research Program
Chapter 3
Chapter 3
Notes and
References
Richard P. Hallion, editor, The Hypersonic Revolution: Case Studies in the History of Hypersonic Technology
(Aeronautical Systems Division, Wright-Patterson AEB, Ohio, 1987), Volume I, Transiting from Air to Space: The
North American X-15, p. 129.
James E. Love, "History and Development of the X-15 Research Aircraft," not dated, DFRC History Office, p. 12.
Because the NB-52 never left the ground, this attempt does not have a program flight number.
James E. Love, "History and Development of the X-15 Research Aircraft," not dated, DFRC History Office, p. 13.
Ibid.
The configuration of the X-15 included a short fixed vertical stabilizer with an all-moving rudder above it on top of
the fuselage, and short fixed ventral stabilizer with a jettisonable all-moving rudder below it This lower rudder will
be called the ventral rudder for simplicity and clarity.
Fuselage stations are measured in inches from a fixed point on the nose of the aircraft.
A. Scott Crossfield, during an interview in the NBC documentary film The Rocket Pilots, 1989.
James E. Love, "History and Development of the X-15 Research Aircraft," not dated, DFRC History Office, p. 14.
Ibid.
Concerns over panel flutter resulted in extensive redesign of the proposed X-20 Dyna-Soar.
James E. Love, "History and Development of the X-15 Research Aircraft," not dated, DFRC History Office, p. 14.
The XLR99 was popularly considered to be a million horsepower engine. By the definition in Websters, the horse-
power of a rocket engine is determined by multiplying the thrust (in pounds) times the speed (in mph), divided by 375.
Therefore, the XLR99 would be 57,000 lbf * 4,520 mph / 375 = 687,040 hp. Not quite a million, but still impressive
for an 800 pound engine.
James E. Love, "History and Development of the X-15 Research Aircraft," not dated, DFRC History Office, p. 17.
Ibid., pp. 15-16.
X-15 Research Airplane Flight Record, North American Aviation, NA-65-1, revised 15 May 1968.
James E. Love, "History and Development of the X-15 Research Aircraft," not dated, DFRC History Office, p. 21.
Email from Peter W. Merlin, DFRC History Office, 18 November 1999.
James E. Love, "History and Development of the X-15 Research Aircraft," not dated, DFRC History Office, p. 20.
A. Scott Crossfield and Clay Blair, Always Another Dawn: The Story of a Rocket Test Pilot, World Publishing Co.,
1960, pp. 307-366.
Achieved by Captain Milburn Apt in the first BeE X-2 on 27 September 1956. Unfortunately, Apt was killed on this flight.
Wendell H. Stillwell, X-15 Research Results, (Scientific and Technical Information Branch, NASA, Washington, DC:
NASA SP-60, 1965), pp. 65.
John V. Becker, "The X-15 Program in Retrospect" (paper presented at the 3rd Eugen Sanger Memorial Lecture,
Bonn, Germany, 4-5 December 1968); and James E. Love, X-15: Past and Future, paper presented to the Fort Wayne
Section, Society of Automotive Engineers, 9 December 1964.
Wendell H. Stillwell, X-15 Research Results, (Scientific and Technical Information Branch, NASA, Washington, DC:
NASA SP-60, 1965), pp. 51-52.
Wendell H. Stillwell, X-15 Research Results, (Scientific and Technical Information Branch, NASA, Washington, DC:
NASA SP-60, 1965). p. iv; see also Walter C. Williams, "The Role of the Pilot in the Mercury and X-15 Flights" (in
the Proceedings of the Fourteenth AGARD General Assembly, 16-17 September 1965, Portugal).
James E. Love, "History and Development of the X-15 Research Aircraft," not dated, DFRC History Office, p. 17.
James E. Love, and William R. Young, Survey of Operation and Cost Experience of the X-15 Airplane as a Reusable
Space Vehicle (Washington, DC: NASA TN-D-3732, 1966).
James E. Love, "History and Development of the X-15 Research Aircraft," not dated, DFRC History Office, p. 23.
Memorandum from Homer Newell to Hugh L. Dryden, 18 December 1961, subject: X-15 follow-on program;
Memorandum from Paul F. Bikle to Hartley Souls' (probably November 1961); NASA news release 61-261. All in the
files of the NASA Dryden History Office.
Air Force Systems Command, X-15 System Package Program, 6-37-48; NASA news release 62-98; X-15 news
release 62-91; Letter from Hugh L. Dryden to Lieutenant General James Ferguson, 15 July 1963; NASA news release
64-42. All in the files of the NASA Dryden History Office.
John V. Becker, "A Hindsight Study of the NASA Hypersonic Rocket Engine Program" (Washington DC,
NASA/OAST, 1 July 1976), p. 9, in the files of the NASA History Office.
Memorandum from Paul F. Bikle to the NASA Office of Aeronautical Research, subject: Repairs to the X-15-2
Airplane, 27 December 1962, in the files of the NASA History Office.
Ronald G. Boston, "Outline of the X-15's Contributions to Aerospace Technology," typescript available in the NASA
Dryden History Office, p. 12.
Robert A. Hoover and Robert A. Rushworth, "X-15A-2 Advanced Capability" (a paper presented at the annual sym-
posium of The Society of Experimental Test Pilots, Beverly Hills, California, 25-26 September 1964).
Cockpit voice transcription for Rushworth flight, 14 August 1964; X-15 Operations Flight Report, 19 August 1964;
Rushworth flight comments, not dated. All in the files of the NASA Dryden History Office.
These included contracts from both the Air Force and various NASA centers as part of the Integral Launch and
Reentry Vehicle (ILRV) programs.
Dennis R. Jenkins, The History of Developing the National Space Transportation System: The Beginning through
STS-75 (second edition; Cape Canaveral, Florida: Dennis R. Jenkins, 1997), p. 129.
Robert A. Hoover and Robert A. Rushworth, "X-15A-2 Advanced Capability" (a paper presented at the annual sym-
posium of The Society of Experimental Test Pilots, Beverly Hills, California, 25-26 September 1964).
William J. Knight, "Increased Piloting Tasks and Performance of X-15A-2 in Hypersonic Flight" (a paper presented
at the annual symposium of the SETP, Beverly Hills, CA, 28-30 September 1967).
Flight Plan for Flight Number 2-53-97, 19 September 1967.
64
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 3
The Flight Research Program
1 James R. Welsh, "Preliminary Report on X-15 Flight 2-53-97" (X-15 Planning Office, 26 October 1967).
! John V. Becker, "The X-15 Program in Retrospect" (paper presented at the 3rd Eugen Sanger Memorial Lecture,
Bonn, Germany, 4-5 December 1968).
' Interview with Jack Koll, 28 February 1977, (interviewer unknown), in the files of the AFSC History Office.
1 Dennis R. Jenkins, The History of Developing the National Space Transportation System: The Beginning Through
STS-75 (second edition; Cape Canaveral, Florida: Dennis R. Jenkins, 1997), p. 108.
' Ronald G. Boston, "Outline of the X-15's Contributions to Aerospace Technology," typescript available in the NASA
Dryden History Office, p. 8.
s Major Michael J. Adams, Captain Joseph H. Engle, Major William J. "Pete" Knight, Lieutenant Colonel Robert A.
Rushworth, and Major Robert M. White.
7 To date, this is the only hypersonic spin that has been encountered during manned flight research. See Bellman,
Donald R., et al., Investigation of the Crash of the X-15-3 Aircraft on November 15, 1967, January 1968, pp. 8-15.
8 Ibid.
'Ibid.
"Ibid.
1 Memorandum from John V. Becker to Floyd Thompson, 29 October 1964; Letter from Paul F. Bikle to C. W. Harper,
13 November 1964.
2 USAF Headquarters Development Directive No. 32, 5 March 1964, reprinted in X-15 System Package Program, 13-7.
3 Becker, John V. and Supp, R. E., Report of Meeting of USAF/NASA Working Groups on Hypersonic Aircraft
Technology, 21-22 September 1966.
4 Love, James E. and Young, William R., NASA TN D-3732, Survey of Operation and Cost Experience of the X-15
Airplane as a Reusable Space Vehicle, November 1966, pp. 7.
Jack McKay was seri-
ously injured on Flight
2-31-52, 9 November
1962.TheXLR99
stuck at 35 percent
thrust, forcing McKay
to abort. The flaps did
not extend fully, result-
ing in a fast landing
on Mud Lake. The air-
craft rolled over after
touchdown. McKay
recovered and came
back to fly the X-15 22
more times. (NASA
photo E-91 49)
The X-15A-2 being
prepared for Flight
2-43-75 on 3
November 1965. This
was the first flight with
the external propellant
tanks, which were
empty. The tanks were
painted bright orange
and white to aid in
photography during
separation. (NASA)
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
65
The Flight Research Program
Chapter 3
vW-A™^'
s, ^ *»#*'<*; v :
Ate, ~ ' •
* *vr> #)
All three X-1 5s are
lined up in the main
hangar at the Flight
Research Center in
1966. Note the lifting
bodies in the back-
ground, along with an
F-4A, F5D, and DC-3.
(NASA photo
EC66-1461)
Six of the twelve men
to fly X-1 5 pose for a
portrait in 1966 Left to
right): Captain Joseph
H. Engle (USAF),
Major Robert A.
Rushworth (USAF),
John B. "Jack" McKay
(NASA), William J.
"Pete" Knight (NASA),
Milton O. Thompson
(NASA), and William
H. Dana (NASA).
(NASA Photo
EC66-1017)
66
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 4
The Legacy of the XA5
Chapter 4
The Legacy of the X-15
The year 1999 marked the 40th anniversary of
the first flight of the X-15; this anniversary
occurred more than 30 years after the program
ended. The X-15 was the last high-speed
research aircraft to fly as part of the research
airplane program. The stillborn X-30 of the
1980s never took flight, and the verdict is still
out on the fate of the Lockheed Martin X-33
demonstrator. Neil Armstrong, among others,
once called the X-15 "the most successful
research airplane in history." 1
Twelve men flew X-15. Scott Crossfield was
first; William Dana was last. Pete Knight
went 4,520 mph (Mach 6.70); Joe Walker
went 67 miles (354,200 feet) high. Five of
the pilots were awarded Astronaut Wings.
Mike Adams died. What was learned? What
should have been learned?
In October 1968 John V. Becker enumerated
22 accomplishments from the research and
development work that produced the X-15,
28 accomplishments from its actual flight
research, and 16 from experiments carried by
the X-15. Becker's comments have been well
documented elsewhere, but are quoted here
as appropriate. 2
Nearly ten years after Becker's assessment,
Captain Ronald G. Boston of the U.S. Air
Force Academy's history department
reviewed the X-15 program for "lessons
learned" that might benefit the development
of the X-24C National Hypersonic Flight
Research Facility Program, an effort that was
cancelled shortly afterwards. Boston's paper
offered an interesting perspective on the X-15
from the vantage point of the mid-1970s. 3
In 1999, the historian at the Dryden Flight
Research Center, J. D. "Dill" Hunley, wrote
a lessons-learned paper on the X-15.
Drawing heavily but not uncritically upon
Becker's and Boston's insights, it too pro-
vides an interesting perspective, and is quot-
ed several times in the pages that follow. 4
Lessons Learned (or not)
The X-15 was designed to achieve a speed of
Mach 6 and an altitude of 250,000 feet to
explore the hypersonic and near-space envi-
ronments. More specifically, its goals were:
(1) to verify existing (1954) theory and
wind tunnel techniques;
(2) to study aircraft structures under high
(1,200 degrees Fahrenheit) heating;
(3) to investigate stability and control
problems associated with high-altitude
boost and reentry; and
(4) to investigate the biomedical effects of
both weightless and high-g flight.
All of these design goals were met, and most
were surpassed. The X-15 actually achieved
Mach 6.70, 354,200 feet, 1,350 degrees
Fahrenheit, and dynamic pressures over
2,200 pounds per square foot. 5 In addition,
once the original research goals were
achieved, the X-15 became a high-altitude
hypersonic testbed for which 46 follow-on
experiments were designed.
Unfortunately due to the absence of a subse-
quent hypersonic mission, aircraft applica-
tions of X-15 technology have been few.
Given the major advances in materials and
computer technology in the 30 years since
the end of the flight research program, it is
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
67
The Legacy of the X-J5
Chapter 4
unlikely that many of the actual hardware
lessons are still applicable. That being said,
the lessons learned from hypersonic model-
ing, simulation, and the insight gained by
being able to evaluate actual X-15 flight test
results against wind tunnel and predicted
results, greatly expanded the confidence of
researchers during the 1960s and 1970s.
In space, however, the X-15 contributed sig-
nificantly to both the Apollo and Space
Shuttle programs. Perhaps the major contribu-
tion was the final elimination of a spray-on
ablator as a possible thermal protection sys-
tem for the Space Shuttle. This would likely
have happened in any case as the ceramic tiles
and metal shingles were further developed,
but the operational problems encountered
with the (admittedly brief) experience on
X-15A-2 hastened the departure of the abla-
tors. Although largely intangible, proving the
value of man-in-the-loop simulations and pre-
cision "dead-stick" landings have also been
invaluable to the Space Shuttle program.
The full value of X-15's experience to
designing advanced aircraft and spacecraft
can only be guessed at. Many of the engi-
neers (including Harrison Storms) from the
X-15 project worked on the Apollo space-
craft and the Space Shuttle. In fact, the X-15
experience may have been part of the reason
that North American was selected to build
later spacecraft. Yet X-15's experience is
overshadowed by more recent projects and
becomes difficult to trace as systems evolve
through successive programs. Nonetheless,
many of those engineers are confident that
they owe much to the X-15, even if many are
at a loss to give any concrete examples.
Political Considerations
John V. Becker, arguably the father of the
X-15, once stated that the project came along
at " ... the most propitious of all possible
times for its promotion and approval." At the
time it was not considered necessary to have
a defined operational program in order to
conduct basic research. There were no
"glamorous and expensive" manned space
projects to compete for funding, and the gen-
eral feeling within the nation was one of try-
ing to go faster, higher, or further. In today's
environment, as in 1968 when Becker was
commenting, it is highly unlikely that a pro-
gram such as the X-15 could gain approval. 6
This situation should give pause to those who
fund aerospace projects solely on the basis of
their presumably predictable outcomes and
their expected cost effectiveness. Without the
X-15's pioneering work, it is quite possible
that the manned space program would have
been slowed, conceivably with disastrous
consequences for national prestige. 7
According to Becker, proceeding with a gen-
eral research configuration rather than with a
prototype of a vehicle designed to achieve a
specific mission as envisioned in 1954 was
critical to the ultimate success the X-15
enjoyed. Had the prototype route been taken,
Becker believed that "... we would have
picked the wrong mission, the wrong struc-
ture, the wrong aerodynamic shapes, and the
wrong propulsion." He also believed that a
second vital aspect to the success of the X- 15
was its ability to conduct research, albeit for
very short periods of time, outside the sensi-
ble atmosphere. 8
The latter proved to be the most important
aspect of X-15 research, given the contribu-
tions it made to the space program. But in
1954 this could not have been foreseen. Few
people then believed that flight into space
was imminent, and most thought that flying
humans into space was improbable before
the next century. Fortunately, the hypersonic
aspects of the proposed X-15 enjoyed "virtu-
ally unanimous approval," although ironical-
ly the space-oriented results of the X-15 have
been of greater value than its contributions to
aeronautics. 9
A final lesson from the X-15 program is that
success comes at a cost. It is highly likely that
researchers can never accurately predict the
costs of exploring the unknown. If you under-
68
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 4
The Legacy of the X-15
stand the problems well enough to accurately
predict the cost, the research is not necessary.
The original cost estimate for the X-15 pro-
gram was $10.7 million. Actual costs were
still a bargain in comparison with those for
Apollo, Space Shuttle, and the International
Space Station, but at $300 million, they were
over almost 30 times the original estimate. 10
Because the X-15's costs were not subjected
to the same scrutiny from the Administration
and Congress that today's aerospace projects
undergo, the program continued. One of the
risks when exploring the unknown is that you
do not understand all the risks. Perhaps politi-
cians and administrators should learn this par-
ticular lesson from this early and highly suc-
cessful program.
Rocket Engines
The XLR99 was the first large man-rated
rocket engine that was capable of being
throttled and restarted in flight. This com-
plexity resulted in many aborted missions
(approximately one-tenth of all mission
aborts) and significantly added to the devel-
opment cost of the engine. When the X-15
program ended, many felt that the throt-
tleable feature might have been a needless
luxury that complicated and delayed the
development of the XLR99.
But in the mid-1960s these attributes were
considered vital to the development of a
rocket engine to power the Space Shuttle. At
the time, Shuttle was to consist of two total-
ly reusable stages — essentially a large hyper-
sonic aircraft that carried a smaller winged
spacecraft much like the NB-52s carried the
X-15s. The same basic engine was going to
power both stages; the pilots therefore need-
ed to be able to control its thrust output. At
some points in the early Shuttle concept
development phases, the same engines
would also be used on-orbit to effect changes
in the orbital plane. So the original concept
for the Space Shuttle Main Engines (SSME)
included the ability to operate at 10 percent
of their rated thrust, and to be restarted mul-
tiple times during flight."
In the end, the production SSMEs are throt-
tleable within much the same range as the
XLR99 — 65 to 109 percent, in one percent
increments. In actuality about the only rou-
tine use of this ability is to throttle down as
the vehicle reaches the point of maximum
dynamic pressure during ascent, easing
stresses on the vehicle for a few seconds on
each flight. Even this would not have been
necessary with a different design for the
solid rocket boosters. 12 So the complexities
required to enable the engine to throttle may,
again, have been a needless luxury.
Nevertheless, the development pains experi-
enced by Reaction Motors provided insight
for Pratt & Whitney and Rocketdyne (the
two main SSME competitors) during the
design and development of the SSMEs.
Human Factors
Coining at a time when serious doubts were
being raised concerning man's ability to han-
dle complex tasks in the high-speed, weight-
less environment of space, the X-15 became
the first program for repetitive, dynamic mon-
itoring of pilot heart rate, respiration, and
EKG under extreme stress over a wide range
of speeds and forces. The Bioastronautics
Branch of the AFFTC measured unusually
high heart and breathing rates on the parts of
the X-15 pilots at points such as launch of the
X-15 from the NB-52, engine shutdown, pull-
out from reentry, and landing. Heart rates
averaged 145 to 160 beats per minute with
peaks on some flights of up to 185 beats per
minute. Despite the high levels, which caused
initial concern, these heart rates were not
associated with any physical problems or loss
of ability to perform piloting tasks requiring
considerable precision. Consequently, theo-
retical limits had to be re-evaluated, and
Project Mercury as well as later space pro-
grams did not have to be concerned about
such high heart rates in the absence of other
symptoms. In fact, the X-15's data provided
some of the confidence to go ahead with early
manned Mercury flights — the downrange bal-
listic shots being not entirely dissimilar to the
X-15's mission profile. 13
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
69
TheLegacyoftheX45
Chapter 4
The bio-instrumentation developed for the
X-15 program has allowed similar monitor-
ing of many subsequent flight test programs.
Incorporated into the pressure suit, pickups
are unencumbering and compatible with air-
craft electronics. The flexible, spray-on wire
leads have since found use in monitoring car-
diac patients in ambulances.
Another contribution of the X-15 program
was the development of what John Becker
calls the "first practical full-pressure suit for
pilot protection in space." 14 The David Clark
Company had worked with the Navy and the
HSFS on an early full-pressure suit for use in
high-altitude flights of the Douglas D-558-
U; the suit worn by Marion Carl on his high-
altitude flights was the first step. This suit
was made of a waffle-weave material and
had only a cloth enclosure rather than a hel-
met. It should be noted that Scott Crossfield
was heavily involved in the creation of this
suit, the success of which Crossfield attrib-
utes to "... David Clark's genius." 15
The David Clark Company later developed
the A/P-22S-2 pressure suit that permitted a
higher degree of mobility. 16 It consisted of a
link-net material covering a rubberized pres-
sure garment. Developed specifically for the
X-15, the basic pressure suit provided part of
the technological basis for the suits used in
the Mercury and Gemini programs. It was
later refined as the A/P-22S-6 suit that
became the standard Air Force operational
suit for high altitude flight in aircraft such as
the U-2 and SR-71. However, it should be
added that the space suit for Project Mercury
underwent further development and was pro-
duced by the B.F. Goodrich Company rather
than the David Clark Company, so the line of
development from X-15 to Mercury was not
entirely a linear one, and security surround-
ing the U-2 and Blackbird programs have
obscured some of this history. 17
X-15 pilots practiced in a ground-based sim-
ulator that included the X-15 cockpit with all
of its switches, controls, gauges, and instru-
ments. An analog computer converted the
pilot's movements with the controls into
instrument readings and indicated what the
aircraft would do in flight to respond to con-
trol actions. After a flight planner had used
the simulator to lay out a flight plan, the pilot
and flight planner worked "for days and
weeks practicing for a particular flight." The
X-15 simulator was continually updated with
data from previous flights to make it more
accurate, and eventually a digital computer
allowed it to perform at higher fidelity. 18
Much has been made of the side-stick con-
troller used on the X-15. Although the con-
cept has found its way onto other aircraft, it
has usually been for reasons other than those
that initially drove its use on the X-15. The
X-15 designers feared that the high g-loads
encountered during acceleration would make
it impossible for the pilot to use the conven-
tional center stick; such worries are not the
reason Airbus Industries has used the con-
troller on the A318-series airliners. And
although the side-stick controller has proven
very popular in the F-16 fighter, it has not
been widely adopted. Nevertheless, the X-15
experience provided a wealth of data over a
wide range of flight regimes.
Some phases of X-15 flight, such as reentry,
were marginally stable, and the aircraft required
artificial augmentation (damping) systems to
achieve satisfactory stability. The X-15 necessi-
tated the development of an early stability aug-
mentation system (SAS). The first two X-15s
were equipped with a simple fail-safe, fixed-
gain system. The X-15-3 was equipped with a
triple-redundant adaptive flight control system;
the pilot flew via inputs to the augmentation
system. Although a point of continuing debate,
the X-15 did not incorporate a "fly-by-wire"
system if meant to denote a nonmechanically
linked control system. Nevertheless, the SAS
system did "fly" the X-15-3 based on pilot input
rather than the pilot flying it directly. This basic
concept would find use on an entire generation
of aircraft, including such high performance
fighters as the F-15. The advent of true fly-by-
wire aircraft, such as the F/A-18, would
advance the concept even further.
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Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 4
The Legacy of the X' 15
Aeronautics
In 1954, the few existing hypersonic wind
tunnels were small and presumably unable to
simulate the conditions of actual flight at
speeds above Mach 5. The realistic fear at
the time was that testing in them would fail
to produce valid data. The X-15 provided the
earliest, and so far most significant, valida-
tion of hypersonic wind tunnel data. This
was of particular significance since it would
be extremely difficult and very expensive to
build a large-scale hypersonic wind tunnel.
occurred over most surfaces. Small surface
irregularities, which produced turbulent flow
at transonic and supersonic speeds, also did
so at Mach 6. 20 Thus, engineers had to aban-
don their hopeful expectations. Importantly,
X-15 flight test data indicated that hyperson-
ic flow phenomena were linear above Mach
5, allowing increased confidence during
design of the Space Shuttle, which must rou-
tinely transition through Mach 25 on its way
to and from space. The basic X-15 data were
also very useful to the NASP designers while
that program was viable.
This general validation, although broadly con-
firmed by other missiles and spacecraft, came
primarily from the X-15; it made the conven-
tional, low-temperature, hypersonic wind tun-
nel an accepted source of data for configura-
tion development of hypersonic vehicles. 19
The X-15 program offered an excellent
opportunity to compare actual flight data
with theory and wind tunnel predictions. The
X-15 verified existing wind tunnel tech-
niques for approximating interference effects
for high-Mach, high angle-of-attack hyper-
sonic flight, thus giving increased confi-
dence in small-scale techniques for hyper-
sonic design studies. Wind tunnel drag meas-
urements were also validated, except for a 15
percent discrepancy found in base drag —
caused by the sting support used in the wind
tunnel. All of this greatly increased the con-
fidence of engineers as they set about design-
ing the Space Shuttle.
One of the widely held beliefs in the mid-
1950s was the theoretical presumption that
the boundary layer (the thin layer of air close
to the surface of an aircraft) would be highly
stable at hypersonic speeds because of heat
flow away from it. This presumption fostered
the belief that hypersonic aircraft would
enjoy laminar (smooth) airflow over their
surfaces. At Mach 6, even wind tunnel
extrapolations indicated extensive laminar
flow. However, flight data from the X-15
showed that only the leading edges exhibited
laminar flow and that turbulent flow
In a major discovery, the Sommer-Short and
Eckert T-prime aerodynamic heating predic-
tion theories in use during the late 1950s
were found to be 30 to 40 percent in excess
of flight test results. Most specialists in fluid
mechanics refused to believe the data, but
repeated in-flight measurements completely
substantiated the initial findings. This led the
aerodynamicists to undertake renewed
ground-based research to complete their
understanding of the phenomena involved —
highlighting the value of flight research in
doing what Hugh Dryden had predicted for
the X-15 in 1956: that it would "separate the
real from the imagined." 21
Subsequent wind tunnel testing led to
Langley's adopting the empirical Spaulding-
Chi model for hypersonic heating. This
eventually allowed the design of lighter vehi-
cles with less thermal protection that could
more easily be launched into space. The
Spaulding-Chi model found its first major
use during the design of the Apollo com-
mand and service modules and proved to be
quite accurate. In 1999 the Spaulding-Chi
model was still the primary tool in use.
Based on their X-15 experience, North
American devised a computerized mathe-
matical model for aerodynamic heating
called HASTE (Hypersonic and Supersonic
Thermal Evaluation) which gave a workable
"first cut" approximation for design studies.
HASTE was, for example, used directly in
the initial Apollo design study. Subsequent
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71
The Legacy of the X' 15
Chapter 4
versions of this basic model were also used
early in the Space Shuttle design evolution.
At the time of the first Mach 5 X-15 flight,
perhaps its greatest contribution to aeronau-
tics was to disprove the existence of a "sta-
bility barrier" to hypersonic flight that was
suspected after earlier research aircraft
encountered extreme instability at high
supersonic speeds. Although of Utile conse-
quence today, the development of the
"wedge" tail allowed the X-15 to successful-
ly fly above Mach 5 without the instability
that had plagued the X-l series and X-2 air-
craft at much lower speeds. The advent of
modern fly-by-wire controls and stability
augmentation systems based around high
speed digital computers have allowed
designers to compensate for gross instabili-
ties in basic aerodynamic design, and even to
tailor an aircraft's behavior differently for
different flight regimes. The era of building a
vehicle that is dynamically stable has passed,
and with it much of this lesson.
The art of simulation grew with the X-15 pro-
gram, not only for pilot training and mission
rehearsal, but for research into controllability
problems. The same fixed-based simulator
used by the pilots could also be used to
explore those areas of the flight envelope
deemed too risky for actual flight. The X-15
program showed the value of combining wind
tunnel testing and simulation in maximizing
the knowledge gained from each of the 199
test flights. It also provided a means of com-
paring "real" flight data with wind tunnel
data. It is interesting to note that the man-in-
the-loop simulation first used on X-15 found
wide application on the X-30 and the X-33. In
fact, DFRC research pilot Stephen D. Ishmael
has flown hundreds of hours "in" the X-33,
which ironically is an unpiloted vehicle.
Flight Research and Space Flight
Before the X-15, high-speed research air-
craft flown at Edwards could be monitored
and tracked from Edwards. The trajectory of
the X-15 extended much farther from
Edwards than those of the previous research
aircraft, requiring two up-range stations
where tracking, communications, and
telemetry equipment were installed and inte-
grated with the control room back at the
FRC. Along the X-15 flight route, program
personnel also surveyed a series of dry
lakebeds for emergency landings and tested
them before each flight to ensure they were
hard enough to permit the X-15 to land. 22 In
many ways this parallels the tracking and
communications network and the transat-
lantic abort sites used by the Space Shuttle.
The opportunity to observe pilot perform-
ance under high stress and high g-forces
indicated that an extensive ground training
program was needed to prepare pilots to han-
dle the complex tasks and mission profiles of
space flight. The result was a simulation pro-
gram that became the foundation for crew
training for all human space flight. The pro-
gram depended on four types of simulation.
Prior to the first X-15 mission, the abil-
ity of the pilot to function under the high
g-forces expected during launch and
reentry was tested in a closed-loop, six-
degree-of-freedom centrifuge at the
Naval Air Development Center,
Johnsville, Pennsylvania. This project
became the prototype for programs set
up at the Ames Research Center and the
Manned Spacecraft Center at Houston
(now the Johnson Space Center). 23
A static cockpit mockup provided the
means for extensive mission rehearsal —
averaging 20 hours per 10 minute flight.
Such preparation was directly responsi-
ble for the high degree of mission success
achieved as pilots rehearsed their pri-
mary, alternate, and emergency mission
profiles. Similar, but much more elabo-
rate, rehearsals are still used by astro-
nauts preparing for Space Shuttle flights.
X-15 pilots maintained proficiency by
flying an NT-33 or JF-100C variable-
stability aircraft whose handling charac-
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Chapter 4
The Legacy of the X' 15
A great deal of what
was learned on X-15
went on to build
Space Shuttle.
(NASA)
teristics could be varied in flight, simu-
lating the varied response of the X-15
traversing a wide range of velocities and
atmospheric densities. Much of this
training is now conducted in advanced
motion-based simulators, although the
Air Force still operates a variable-stabil-
ity aircraft (the VISTA F-16).
Pilots practiced the approach and land-
ing maneuver in F-104 aircraft. With
landing gear and speed brakes extended,
the F-104's power-off glide ratio
approximated that of the unpowered
X-15. Shuttle crews continue this same
practice using modified Gulfstream
Shuttle Training Aircraft (STA).
Astronaut "capsule communicators," (cap-
comms) were a direct outgrowth of the X-15's
practice of using an experienced pilot as the
ground communicator for most X-15 mis-
sions. 24 This practice existed through Mercury,
Gemini, and Apollo, and continues today on
Space Shuttle missions. It is still believed that
a pilot on the ground makes the best person to
communicate with the crew, especially in
stressful or emergency situations. 25
Subsequent flight test work at Edwards
relied heavily on the methodology developed
for the X-15. There are no fewer than three
high-tech control facilities located at
Edwards today; the facility at Dryden, the
Riddley Control Center complex at the
AFFTC, and the B-2 control complex locat-
ed on South Base. Each of these control cen-
ters has multiple control rooms for use dur-
ing flight test. The X-33 program has built
yet another control room, this one located
near the launch site at Haystack Butte. 26
The X-15 program required a tracking net-
work known as "High Range." Operational
techniques were established for real-time
flight monitoring which were carried over to
the space program. The experience of setting
up this control network became something of
a legacy to Mercury and later space projects
through the personnel involved. Gerald M.
Truszynski, as Chief of the Instrumentation
Division at the FRC, had participated in set-
ting up the High Range, as had Edmond C.
Buckley, who headed the Instrument
Research Division at Langley. The Tracking
and Ground Instrumentation Group at
Langley had the responsibility for tracking
the Mercury capsules, and it was headed,
briefly, by Buckley. 27
Buckley soon transferred to NASA
Headquarters as assistant director for space
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The Legacy of the X'15
Chapter 4
flight operations, with Truszynski joining him
in 1960 as an operations engineer. Both con-
tinued to be involved in instrumentation and
communication until a reorganization under
NASA Administrator James Webb created an
Office of Tracking and Data Acquisition
with Buckley as director. Buckley named
Truszynski as his deputy, and in 1962
appointed him to lead the Apollo Task Group
that shaped the Apollo tracking and data net-
work. 28 Much of this same infrastructure was
used early in the Space Shuttle program.
Meanwhile, Walter Williams, who had headed
the NACA operations at the HSFS/FRC since
1946, was reassigned as Associate Director of
the newly formed Space Task Group at
Langley in September 1959. He eventually
served as the Director of Operations for
Mercury, and then as Associate Director of the
Manned Spacecraft Center. He also served as
operations director in the Mercury Control
Center at Cape Canaveral during the Mercury
flights of Alan Shepard, Gus Grissom, and
John Glenn in 1961 and 1962. 29
Experience from the NASA 1 control room
undoubtedly influenced the development of
the Mercury Control Center at Cape
Canaveral, and perhaps more distantly, even
the Mission Control Center (MCC) 30 at
Houston. 31 However, the spacecraft control
rooms and their tracking and data acquisition
systems drew on many other sources (includ-
ing the missile ranges which they shared), 32
although the experience setting up the High
Range and operating the NASA 1 control
room undoubtedly provided some opera-
tional perspectives.
An often overlooked area where the X-15
influenced Space Shuttle operations is in the
energy management maneuvers immediately
prior to landing. By demonstrating that it
was possible to make precision unpowered
landings with vehicles having a low lift-
over-drag ratio, the X-15 program smoothed
the path for the slightly later lifting-body
program and then for the space shuttle pro-
cedures for energy management and landing.
The techniques used by X-15 pilots consist-
ed of arriving at a "high key" above the
intended landing point. Once he reached the
high key, the pilot did not usually need or
receive additional information from the con-
trol room; he could complete the landing
using visual information and his own experi-
ence with practice landings in an F- 104 con-
figured to simulate an X-15 landing. With
considerable variation on different missions,
the pilot would arrive at the high key on an
altitude mission at about 35,000 feet, turn
180 degrees and proceed to a "low key" at
about 18,000 feet, where he would turn
another 180 degrees and proceed to a landing
on Rogers Dry Lake. Depending upon the
amount of energy remaining, the pilot could
use shallow or tight bank angles and speed
brakes as necessary.
Because of their much higher energy, the
standard approach for the Space Shuttle con-
sists of a variation on this 360-degree
approach. As a Shuttle approaches the run-
way for landing, if it has excess energy for a
normal approach and landing, it dissipates
this energy in S-turns (banking turns) until it
can slow to a subsonic velocity at about
49,000 feet of altitude some 25 miles from
the runway. It then begins the approach and
landing phase at about 10,000 feet and an
equivalent airspeed of about 320 mph some 8
miles from the runway. 33 Early in the Space
Shuttle program, a specially-configured
T-38 34 would accompany the orbiter on the
final approach, much as the X-15 chase air-
craft did at Edwards. Shuttle pilots practice
in a specially-configured Gulfstream Shuttle
Training Aircraft, much as the X-15 pilots
did in the modified F-104.
Components and Construction
The X-15 was designed with a hot-structure
that could absorb the heat generated by its
short-duration flight. Remember, the X-15
seldom flew for over ten minutes at a time,
and a much shorter time was spent at the
maximum speed or dynamic pressure.
Development showed the validity of ground
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Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 4
The Legacy of the X-15
"partial simulation" testing of primary mem-
bers stressed under high temperature. A
facility was later built at DFRC for heat-
stress testing of the entire structure, and sim-
ilar testing was accomplished on the YF-12A
Blackbird and the Space Shuttle structural
test article (STA-099). 35
The X-15 pioneered the use of corrugations
and beading to relieve thermal expansion
stresses. Metals with dissimilar expansion
coefficients were also used to alleviate stress-
es, and the leading edges were segmented to
allow for expansion. Around the same time,
similar techniques were apparently developed
independently by Lockheed for use on
Blackbird series of Mach 3+ aircraft.
The X-15 represented the first large-scale
use of Inconel X, in addition to extensive use
of titanium alloys. This required the develop-
ment of new techniques for forming, milling,
drilling, and welding that came to be widely
used in the aerospace industry. North
American pioneered chemical milling, a
construction technique that has since been
used on other projects.
The differentially deflected horizontal stabi-
lizers on the X-15 provided roll and pitch
control and allowed designers to eliminate
the ailerons that would have provided a
severe structural and theromodynamic prob-
lem within the thin wing section used on the
X-15. This configuration was already
being flight tested by less exotic aircraft
(YF-107A) at the same time it was used on
the X-15, but nevertheless proved extremely
valuable. It is common practice today to use
differential stabilators on modern aircraft,
particularly fighters, although in most cases
conventional ailerons are also retained; the
flight control system deciding when to use
which control surfaces based on conditions.
The all-moving vertical surfaces in lieu of
conventional rudders has proven somewhat
less attractive to aircraft designers. North
American used an all-moving vertical sur-
face on the A-5 Vigilante, designed not long
after the X-15. Lockheed also used all-mov-
ing surfaces on the Blackbird series of Mach
3 aircraft, although it is difficult to ascertain
if the X-15 influenced this design choice.
The X-15 designers also had to solve prob-
lems relating to high aerodynamic heating in
proximity to cryogenic liquids. This led to
cryogenic tubing that was used on parts of
the Apollo spacecraft, and thermal insulation
design features that were later used on the
Space Shuttle. An early experience of run-
ning a liquid nitrogen cooling line too close
to a hydraulic line taught designers about the
need to fully understand the nature of the flu-
ids they were dealing with. In-flight failures
on high altitude flights with the X-15 also
taught aerospace engineers about such things
as the need to pressurize gear boxes on aux-
iliary power units to prevent foaming of the
lubricant in the low pressure of space, since
that led to material failures. 36
Although the primary structure of the X-15
proved sound, several detailed design prob-
lems were uncovered during early flight tests.
A surprise lesson came with the discovery of
heretofore unconsidered local heating phe-
nomena. Small slots in the wing leading edge,
the abrupt contour change along the canopy,
and the wing root caused flow disruptions that
produced excessive heating and adjacent
material failure. The X-15, tested in "typical"
panels or sections, demonstrated the problems
encountered when those sections are joined
and thus precipitated an analytical program
designed to predict such local heating stress-
es. From this experience, Rockwell engineers
closely scrutinized the segmented carbon-car-
bon composite leading edge of the Space
Shuttle wing. The bimetallic "floating retain-
er" concept designed to dissipate stresses
across the X-15's windshield carried over to
the Apollo and Space Shuttle windshield
designs as well.
On three occasions, excessive aerodynamic
heating of the nose-wheel door scoop caused
structural deformation, permitting hot bound-
ary-layer air to flow into the wheel well,
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75
The Legacy of the X- 1 5
Chapter 4
damaging the landing gear, and in one case
causing the gear to extend at Mach 4.2
(flight 2-33-56). Although the landing gear
remained intact, the disintegration of the tires
made the landing very rough. The need for
very careful examination of all seals became
apparent, and closer scrutiny of surface irreg-
ularities, small cracks, and areas of flow
interaction became routine. The lessons
learned from this influenced the final detailed
design of the Space Shuttle to ensure that
gaps and panel lines were adequately protect-
ed against inadvertent airflow entry.
Other problems from aerodynamic heating
included windshield crazing, panel flutter,
and skin buckling. Arguably, designers could
have prevented these problems through more
extensive ground testing and analysis, but a
key purpose of flight research is to discover
the unexpected. The truly significant lesson
from these problems is that defect in subson-
ic or supersonic aircraft that are compara-
tively minor at slower speeds become much
more critical at hypersonic speeds. 37
One of the primary concerns during the X- 15
development was panel flutter, evidenced by
the closing paper presented at the 1956
industry conference. Panel flutter has proven
difficult to predict at each speed increment
throughout history, and the hypersonic
regime was no different. Although the X- 15
was conservatively designed, and incorporat-
ed all the lessons from first generation super-
sonic aircraft, the fuselage side tunnels and
the vertical surfaces were prone to develop
panel flutter during flight. This led to an
industry-wide reevaluation of panel flutter
design criteria in 1961-62. Stiffeners and
reduced panel sizes alleviated the problems
on the X-15, and similar techniques later
found general application in the high speed
aircraft of the 1960s. 38 The lessons learned at
Mach 6 defined criteria later used in the
development of the Space Shuttle.
The X-15 provided the first opportunity to
study the effects of acoustical fatigue over a
wide range of Mach numbers and dynamic
pressures. In these first in-flight measure-
ments, "boundary layer noise"-related stress-
es were found to be a function of g-force, not
Mach number. Such fatigue was determined
to be ho great problem for a structure
stressed to normal in-flight loading. This
knowledge has allowed for more optimum
structural design of missiles and space cap-
sules that experience high velocities.
On the X-15, the measurement of velocity
was handled by early inertial systems. All
three X-15s were initially equipped with
analog-type systems which proved to be
highly unreliable. Later, two aircraft, includ-
ing the X-15-3 with the adaptive control sys-
tem, were modified with digital systems. In
the subsequent parallel evaluation of analog
versus digital inertial systems, the latter was
found to be far superior. It was far more flex-
ible and could make direct inputs to the
adaptive flight control system; it was also
subject to less error. Thanks to advances in
technology such as laser-ring gyros and dig-
ital computers, inertial systems have become
inexpensive, highly accurate, and very reli-
able. 39 In recent years they have been inte-
grated with the Global Positioning System
(GPS), providing three-dimensional attitude
and position information.
During the early test flights, the X-15 relied
on simple pilot-static pressure instruments
mounted on a typical flight test nose boom.
These were not capable of functioning as
speeds and altitudes increased. To provide
attitude information, the NACA developed
the null-sensing "ball-nose" which could
survive the thermal environment of the X-15.
An extendable pitot tube was added when
the velocity envelope was expanded beyond
Mach 6. Thus far the ball-nose has not found
subsequent application, and probably never
will since inertial and GPS systems have
evolved so quickly. Interestingly, the Space
Shuttle still uses an extendable pitot probe
during the landing phase.
The X-15 was the first vehicle to routinely
use reaction controls. The HSFS had begun
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Chapter 4
The Legacy of the X' 15
research on reaction controls in the mid-
1950s using a fixed-base analog control stick
with a pilot presentation to determine the
effects of control inputs. This was followed
by a mechanical simulator to enable the pilot
to experience the motions created by reac-
tion controls. This device emulated the iner-
tial ratios of the X-1B, which incorporated a
reaction control system using hydrogen-per-
oxide as a monopropellant, decomposed by
passing it through a silver-screen catalyst.
Because of fatigue cracks later found in the
fuel tank of the X-1B, it completed only
three flights using the reaction control sys-
tem before it was retired in 1958. 40
As a result, a JF-104A with a somewhat
more refined reaction control system was
tested beginning in late 1959 and extending
into 1961. The JF-104A flew a zoom-climb
maneuver to achieve low dynamic pressures
at about 80,000 feet that simulated those at
higher altitudes. The techniques for using
reaction controls on the X-15, and more
importantly, for transferring from aerody-
namic controls to reaction controls and back
to aerodynamic controls provided a legacy to
the space program. 41
The X-15-3 was equipped with a Minneapolis
Honeywell MH-96 self-adaptive control sys-
tem designed for the cancelled Dyna-Soar. The
other two X-15s had one controller on the
right-hand side of the cockpit for aerodynamic
controls and another on the left-hand side for
the reaction controls. Thus, the pilot had to use
both hands for control during the transition
from flying in the atmosphere to flying outside
the atmosphere and then back in the opposite
direction. Since there was no static stability
outside the atmosphere, the pilot had to count-
er any induced aircraft motion manually using
the reaction controls. The MH-96 had an atti-
tude hold feature that maintained the desired
attitude except during control inputs. The
MH-96 also integrated the aerodynamic and
reaction controls in a single controller, gready
improving handling qualities during the transi-
tion from aerodynamic to space flight, as well
as reducing pilot workload. 42
But the basic feature of the MH-96 was auto-
matic adjustment of gain (sensitivity) to
maintain a desirable dynamic response of the
airplane. The MH-96 compared the actual
response of the airplane with a preconceived
ideal response in terms of yaw, pitch, and roll
rates. Initially, Milt Thompson stated that the
system was "somewhat unnerving to the
pilot" because he was not in "direct control
of the aircraft" but was only "commanding a
computer that then responded with its own
idea of what is necessary in terms of a con-
trol output." However, pilots became "enthu-
siastic in their acceptance of it" when they
realized that the MH-96 resulted in "more
precise command than was possible" with
the reaction controls by themselves.
Consequently, the X-15-3 with the MH-96
was used for all altitude flights planned
above 270,000 feet. 43
There were some problems with the experi-
mental system, including one that con-
tributed to the death of Mike Adams in X-15-
3 on 15 November 1967. Nevertheless, the
MH-96 constituted a significant advance in
technology that helped pave the way toward
fly-by-wire in the early 1970s. Today, most
every aircraft, and several automobiles, fea-
ture some variation of a fly-by-wire system
with automatic rate-gain adjustment and sta-
bility augmentation. 44
Follow-on Experiments
During the early 1960s, only the X-15 had
the capability to carry a useful payload above
the atmosphere. And unlike missiles, the
X-15 could return equipment and results
for reevaluation, recalibration, and reuse.
Perhaps the earliest true "follow-on" experi-
ment came in 1961: a coating material
designed to reduce the infrared emissions of
the proposed B-70 was tested to Mach 4.43
(525 degrees Fahrenheit) on the exterior sur-
face of an X-15 stabilizer panel. Thus began
a series of 46 follow-on experiments in phys-
ical sciences, space navigation aids, recon-
naissance studies, and advanced aerodynam-
ics. While not all of the 46 experiments were
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77
The Legacy of the X-15
Chapter 4
completed before the X- 15 program ended,
many of them did yield useful data.
Heating : Throughout the X-15's flight career
it participated in heating studies, mainly to
verify the output from wind tunnels and com-
puter simulations. Late in the flight program,
one X-15 was fitted with a sharp leading edge
on the upper vertical stabilizer, and the results
were compared with theory and with data
from the original blunt leading edge.
Astronomy : The ultraviolet stellar photogra-
phy study measured the ultraviolet bright-
ness of several stars to determine their mate-
rial composition. The X-15 carried four cam-
eras on a gimbaled platform in the instru-
ment bay behind the cockpit above the filter-
ing effects of the ozone layer — approximate-
ly 40 miles altitude. Conducted in 1963 and
again in 1966, this work was subsequently
continued on improved sounding rockets,
then on orbital satellites.
The X-15 was ideally suited to measure
atmospheric densities at altitudes of 50,000
to 235,000 feet, cross-checking measure-
ments on ascent with those on descent. Using
the ball-nose to take measurements, flow-
angularity errors were ehminated. The X-15
provided atmospheric seasonal variation
density profiles. Unfortunately, these meas-
urements could only be taken in the area
immediately around Edwards AFB.
The X-15 provided the first direct solar spec-
trum measurement of the Sun from above the
atmosphere. A scientific revelation, this data
allowed the refinement of the Solar Constant
of Radiation which was revalued 2.5 percent
lower than existing ground-based determina-
tions. This constant provides a measure of
thermal energy incident on the Earth and
upon which all photochemical processes
depend. It is also useful for the design of
thermal protection for spacecraft. 45
Micrometeorites : Designed to collect
micrometeorites at various altitudes, this
experiment was part of a larger NASA study
to build a particle-impact data base for
spacecraft design criteria. Only on the last of
six flights did this experiment "catch" any
particles, and those were so contaminated by
the exhaust from the reaction controls that
the project was cancelled.
Space Navigation : The X-15 supported
two — MIT and NASA-Langley — horizon
definition projects to determine the Earth's
infrared horizon radiance profile. This infor-
mation was later used in attitude referencing
systems for orbiting spacecraft. The MIT
work was part of an Apollo support program
seeking alternative means for orbit reinser-
tion guidance in case of radar or communi-
cations failure. The space sextant designed
for this task was checked enroute on Apollo
missions 8, 10, and 1 1 with relatively good
accuracy when compared to radar position.
A successful program to collect data on radi-
ation characteristics of the daytime sky back-
ground was part of an effort to develop a "star
tracking" navigational system. Star trackers
went on to be used aboard U-2 and SR-71 air-
craft, and two of them are mounted in the for-
ward fuselage of each Space Shuttle orbiter. 46
Reconnaissance Systems : The X-15's per-
formance made it an ideal testbed for high-
speed aircraft and satellite reconnaissance
systems. Ultraviolet (UV) sensors were stud-
ied as a means of detecting incoming
ICBMs. This three-part project yielded
promising results, but UV systems were
overshadowed by the more advanced
infrared systems. In an effort to determine
the exhaust plume signature of a typical
rocket exhaust above the ozone layer, the
exhaust plume of the X-15 itself was
scanned. To test the feasibility of detecting a
missile launch by its UV signature, an actual
launch from Vandenberg AFB was scheduled
to be monitored on X-15 flight number 200,
but this never occurred.
Several infrared (IR) satellite detection sys-
tems began as X-15 experiments. As early as
1963, researchers studied the IR exhaust
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Hypersordcs Before the Shuttle — Monographs in Aerospace History Number 1 8
Chapter 4
The Legacy of die X- 15
plume characteristics of the X-15. A follow-
up project to measure the Earth's infrared
background using an IR scanner never
flew before the X-15 program ended.
Nonetheless, the equipment developed for
the project contributed directly to later suc-
cessful tests on U-2 aircraft and thus to the
eventual satellite program.
Optical degradation experiments determined
that the shock wave, boundary-layer flow,
and temperature gradients across windows in
the bottom of the fuselage of X-15A-2
caused negligible degradation to visual,
near-IR, and radar aerial photography to
Mach 5.5 and 125,000 feet.
Ablator Tests : During the early 1960s, the
only practical approach to speeds higher than
Inconel X could withstand appeared to be an
ablative coating of some description, much
as was used on the early space capsules.
Obviously, a better method of applying the
ablator would have to be found, and it would
need to be durable and maintainable. The
material selected for use on the X-15 did not
prove totally successful. Extensive man
power was required to apply and refurbish
the ablator surface, and then the integrity of
the ablator-to-skin bonding was of concern
for subsequent flights. Other operational
problems argued against spray-on ablatives;
the crew could not walk on the vehicle, and
access panels were hard to remove and
recover without leaving surface cracks. Also,
many liquids, including liquid oxygen,
would damage the ablator, requiring a coat
of white paint to seal the ablative material's
surface. The development of workable
ceramic tiles (as used on the Space Shuttle)
and metallic shingles (as proposed for some
early Shuttle concepts; and now for X-33)
have largely negated the need to use ablators.
The short X-15 A operational experience has-
tened the industry away from relying on
ablators for reusable space vehicles.
Hypersonic Research Engine
With little doubt, the most ambitious 47 of the
X-15 experiments was the Hypersonic
Research Engine (HRE) from the Langley
Research Center. At the time that researchers
began to consider supersonic-combustion
ramjet engines during 1954, the X-15 was not
an approved program and played no major
role in the engine's conceptual development.
However, events soon transpired that made
flight testing of a supersonic ramjet engine
desirable, and the Flight Research Center and
Langley proposed a joint project to accom-
plish just that. The 1962 crash of the X-15-2
opened the door for extensive modification
aimed primarily at providing a platform for
development of the Mach 8 air-breathing
HRE. Then, as now, no tunnel facility existed
wherein such an engine could be realistically
tested, and rocket boosters could not give
steady-state tests or return the equipment. 48
The actual prototype engine was to be carried
attached to the lower ventral of the X-15A-2.
Twenty-nine inches were added to the fuse-
lage between the existing tanks for the liquid
hydrogen to power the HRE, two external
fuel tanks were added, and the entire aircraft
was coated with an ablative-type insulator.
During 1965, Garrett- AirResearch was put
under contract to provide six prototype
engines by mid- 1969. As would happen, the
development effort necessary to produce a
workable engine had been severely underesti-
mated, and Garrett quickly ran into problems
that caused serious delays in the project.
In the meantime flight-test evaluations were
made of the modified aircraft itself and of a
dummy HRE attached to the X-15A-2. On
the first and only maximum-speed test of the
X-15A-2 in 1967, shock impingement off the
dummy HRE caused severe heating damage
to the lower empennage, and very nearly
resulted in loss of the aircraft. Though quick-
ly repaired, the X-15A-2 never flew again.
Hindsight would place the blame for this
design oversight on haste and insufficient
flow interaction studies. A key lesson
learned from this episode was not to hang
external stores or pylons on hypersonic air-
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
79
The Legacy of the X-15
Chapter 4
craft, at least not without far more extensive
study of underside flow patterns. As John
Becker later observed, "Flight testing on the
X-15A-2 would have been long-delayed,
hazardous, very costly, and fortunately never
came about." 49
When the X-15 flight program was terminat-
ed, the HRE degenerated into a costly wind
tunnel program using partial-simulation test
models. The HRE was eventually tunnel test-
ed in 1969, and the primary objective of
achieving supersonic combustion was met,
although the thrust produced was less than
the drag created. HRE engineers nonetheless
claim a success in that the objective was
supersonic combustion, not a workable
engine. The program continued until 1975
and never achieved a positive net thrust,
although it still contributed to the technology
base, albeit at a very high cost. A hindsight
study conducted in 1976 concluded that the
HRE's fuel-cooled structure was its main
contribution to future scramjets. 50
Papers Published
Not the least of the technological legacies of
the X-15 consisted of the more than 765
technical documents produced in association
with the program, including some 200
papers reporting on general research that the
X-15 inspired. John Becker saw them as
"confirmation of the massive stimulus and
the focus provided by the [X-15] program." 51
Other Views
William Dana took time in 1987 to write a
paper for the Society of Experimental Test
Pilots pointing out some of the lessons
learned from the X-15 program. 52 Dana
should know — he was the last pilot to fly the
X-15. Two he cited were particularly appro-
priate to the designers of the X-30 and X-33,
although neither heeded the lessons. They
are included here in their entirety:
The first lesson from the X-15 is: Make
it robust. As you have already seen, the
X-15 was able to survive some severe
mistreatment during landings and still
came back to fly another day. The X-15
that broke up after a spinning re-entry
had self-recovered from the spin prior to
break up, and might well have survived
the entire episode had fixed, rather than
self-adaptive, damper gains been used
during re-entry. Another example exists
of where the X-15 did survive a major
stress in spite of operating with a major
malfunction. This flight occurred in
June 1967, when Pete Knight launched
in X-15 No. 1 on a planned flight to
250,000 feet. At Mach 4 and at an alti-
tude of 100,000 feet during the boost,
the X-15 experienced a complete electri-
cal failure that resulted in shutdown of
both auxiliary power units and, there-
fore loss of both hydraulic systems. Pete
was eventually able to restart one of the
auxiliary power units, but not its gener-
ator. By skillful use of the one remain-
ing hydraulic system and the ballistic
controls, Pete was able to ride the X-15
to its peak altitude of 170 or 180,000
feet, reenter, make a 180 degree turn
back to the dry lake at Tonopah, and
dead-stick the X-15 onto the lakebed.
All of these activities occurred without
ever flowing another electron through
the airplane from the time of the original
failure.
There will be a hue and cry from some
that the aerospace plane [X-30 — NASP]
cannot afford the luxury of robustness;
that the aerospace plane, in order to be
able to get to orbit, will have to be highly
weight-efficient and will have to forego
the strength and redundancy margins
which allowed the X-15 to survive during
adversity. And my answer to these people
is: build your first aerospace plane with
X-15 margins, even at the expense of per-
formance; these margins will serve well
while you are learning how to make your
propulsion system operate and learning
how to survive in the heating thicket of
hypersonic flight. Someday, with this
80
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 4
The Legacy of the X'15
knowledge in hand, it will be time to
build a no-margins aerospace plane, but
for now I suggest that you seize all the
margins that you can because you will
need them, as did the X-15.
The other lesson from the X-15 is: con-
duct envelope expansion incrementally.
The typical increment of speed increase
for the original X-15 was about half a
Mach number. With this increment it
was easy to handle the heating damage
that occurred in the original speed
expansion phase. Again, I would expect
to hear protest from the aerospace plane
community, because when using one-
half Mach number increments it is a
long flight test program to Mach 25.
Indeed, I cannot specify what size bite
to take during the aerospace plane enve-
lope expansion, but I can offer you the
X-15A experience, in which two con-
secutive flights carrying the dummy
ramjet were flown to Mach numbers of
4.94 and 6.70. The former flight exhibit-
ed no heat damage because of the wake
of the dummy ramjet; the latter flight
almost resulted in the loss of the aircraft
due to heat damage.
Looking at the X-33 program in particular,
another lesson jumps out. There will only be
a single X-33. The building of three X-15s
allowed the flight test program to proceed
even after accidents. In fact, each of the
X-15s was severely damaged at some time or
another requiring it to be rebuilt. Plus, with
multiple aircraft, it is possible to have one
aircraft down for modification while the oth-
ers continue to fly. And should one aircraft
be lost, as sometimes happens in flight
research, the program can continue. In
today's environment it is highly unlikely that
the X-33 program would continue if it
exploded during an engine test like the
X-15-3 did while ground testing the XLR99.
Hopefully the X-33 will not experience such
a failure, but is that not part of the reason we
conduct flight research — to learn from the
failures as well as the successes?
The New Millennium
As we enter the new millennium, it is inter-
esting to note how the X-15 has shaped aero-
nautics and astronautics. Indeed, when the
X-33 program began during 1996, it was sur-
prising to find that many of the younger con-
tractor engineers were totally unaware of the
X-15, and that most thought the SR-71 was
the fastest aircraft that had ever flown, dis-
counting the Space Shuttle. Interestingly, the
young engineers at Dryden remembered the
program, and when it came to setting up the
instrumentation range (which extends all the
way to the Dakotas), lessons learned from the
X-15 High Range were used. 53
The most obvious difference today has
absolutely nothing to do with the technology
of hypersonic flight. It is the political climate
that surrounds any large project. The NASA
Administrator, Daniel Goldin, told an X-33
all-hands meeting that it was "okay to
fail" — a reference that many times in order
to succeed, you first have to experience prob-
lems that appear to be failures. But this is not
the climate that actually exists. Any failure is
often used as an excuse to cut back or cancel
a project. In most cases the only way to total-
ly avoid failure is to completely understand
what you are doing; but if you completely
understood something, there would be no
point in building an X-plane!
The X-15 is usually regarded as the most suc-
cessful flight research program ever undertak-
en. But the program had its share of failures.
The XLR99 destroyed the X-15-3 before it
had even flown; but the aircraft was rebuilt and
the XLR99 became a very successful research
engine. On several occasions the X-15s made
hard landings, sometimes hard enough to sig-
nificantly damage the aircraft; each time they
were rebuilt and flew again. Mike Adams was
killed in a tragic accident; but less than four
months later William Dana flew the next
research flight. Yes, the X-15 failed often; but
its successes were vastly greater.
Perhaps we have not learned well enough.
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
81
The Legacy of the X' 15
Chapter 4
In the foreword to Milton O. Thompson, At the Edge of Space: the X-15 Flight Program (Washington, DC:
Smithsonian Institution Press, 1992), p. xii.
John V. Becker, "Principal Technology Contributions of X- 15 Program" (NASA Langley Research Center, 8 October
1968), in the files of the NASA History Office.
Ronald G. Boston, "Outline of the X-15's Contributions to Aerospace Technology," typescript available in the NASA
Dryden History Office. For those interested in Boston's original paper, the easiest place to find a copy is in the
Hypersonic Revolution, recently republished by the Air Force History and Museums program. It constitutes the last
section in the X-15 chapter.
J. D. Hunley, "The Significance of the X-15," 1999, an unpublished typescript available at the DFRC History Office.
Boston listed 1,300 degrees Fahrenheit as the maximum temperature, but William Dana reported 1,350 degrees
Fahrenheit in his SETP and AIAA papers. Boston also listed the max-q as 2,000 psf, but in reality it was 2,202 psf on
flight 1-66-111.
John V. Becker, "The X-15 Program in Retrospect" (paper presented at the 3rd Eugen Sanger Memorial Lecture,
Bonn, Germany, 4-5 December 1968), pp. 1-2.
J. D. Hunley, "The Significance of the X-15," 1999, an unpublished typescript available at the DFRC History Office.
John V. Becker, "The X-15 Program in Retrospect" (paper presented at the 3rd Eugen Sanger Memorial Lecture,
Bonn, Germany, 4-5 December 1968), pp. 1-2.
Ronald G. Boston, "Outline of the X-15's Contributions to Aerospace Technology," typescript available in the NASA
Dryden History Office, pp. 12-15.
Kay, W. D., The X-15 Hypersonic Flight Research Program: Politics and Permutations at NASA, in From Engineering
Science to Big Science: The NACA and NASA Collier Trophy Research Project Winners, edited by Pamela E. Mack,
NASA SP-4219, 1998, p. 163.
This concept was finally dropped as the Shuttle development program moved into Phase B. See Dennis R. Jenkins,
The History of Developing the National Space Transportation System: The Beginning through STS-75 (second edition;
Cape Canaveral, Florida: Dennis R. Jenkins, 1997), p. 85.
The still-born advanced solid rocket motors (ASRM) would have had propellant grained shaped to reduce their thrust,
eliminating the need for the SSMEs to be throttled.
Charles J. Donlan, "The Legacy of the X-15" (a paper in the Proceedings of the X-15 30th Anniversary Celebration,
Dryden Flight Research Facility, Edwards, California, 8 June 1989, NASA CP-3105), p. 96; Toll, Thomas A. and
Fischel, Jack, The X-15 Project: Results and New Research, in Volume 2, No. 3 of Astronautics and Aeronautics , p.
24.
John V. Becker, "Principal Technology Contributions of X-15 Program" (NASA Langley Research Center, 8 October
1968), in the files of the NASA History Office.
Letter from A. Scott Crossfield to Dennis R. Jenkins, 30 June 1999.
In the same letter Crossfield points out that "Pressure suit history is very badly chronicled." I definitely found this to
be true, and it is a part of the final X-15 history that needs a great deal of attention paid to it
J. D. Hunley, "The Significance of the X-15," 1999, an unpublished typescript available at the DFRC History Office.
Milton O. Thompson, At the Edge of Space: the X-15 Flight Program (Washington, DC: Smithsonian Institution Press,
1992), pp. 68-71, 154.
John V. Becker, "The X-15 Program in Retrospect" (paper presented at the 3rd Eugen Sanger Memorial Lecture,
Bonn, Germany, 4-5 December 1968), pp. 7-8; John V. Becker, "Principal Technology Contributions of X-15
Program" (NASA Langley Research Center, 8 October 1968), in the files of the NASA History Office.
John V. Becker, "The X-15 Program in Retrospect" (paper presented at the 3rd Eugen Sanger Memorial Lecture,
Bonn, Germany, 4-5 December 1968), pp. 8-9; Braslow, Albert L., Analysis of Boundary-Layer Transition on X-15-2
Research Airplane, NASA TN D-3487, 1966.
John V. Becker, "The X-15 Program in Retrospect" (paper presented at the 3rd Eugen Sanger Memorial Lecture,
Bonn, Germany, 4-5 December 1968), pp. 9-10.
J. D. Hunley, "The Significance of the X-15," 1999, an unpublished typescript available at the DFRC History Office.
Ronald G. Boston, "Outline of the X-15's Contributions to Aerospace Technology," typescript available in the NASA
Dryden History Office, p. 16-17; Milton O. Thompson, At the Edge of Space: the X-15 Flight Program (Washington,
DC: Smithsonian Institution Press, 1992), pp. 70-71.
Ronald G. Boston, "Outline of the X-15's Contributions to Aerospace Technology," typescript available in the NASA
Dryden History Office, p. 18; J. D. Hunley, "The Significance of the X-15," 1999, an unpublished typescript available
at the DFRC History Office.
J. D. Hunley, "The Significance of the X-15," 1999, an unpublished typescript available at the DFRC History Office.
On a corner of the impact range at Edwards.
J. D. Hunley, "The Significance of the X-15," 1999, an unpublished typescript available at the DFRC History Office.
Ibid.
Ibid.
There is often a great deal of confusion between the Mercury Control Center at Cape Canaveral used during the
Mercury and initial Gemini flights, and the Mission Control Center in Houston that has been used since Gemini 5.
Both, confusingly, are abbreviated MCC.
J. D. Hunley, "The Significance of the X-15," 1999, an unpublished typescript available at the DFRC History Office.
Ibid.
Ronald G. Boston, "Outline of the X-15's Contributions to Aerospace Technology," typescript available in the NASA
Dryden History Office, pp. 18-19; Milton O. Thompson, At the Edge of Space: the X-15 Flight Program (Washington,
DC: Smithsonian Institution Press, 1992), pp. 182-186.
Chapter 4
Notes and
References
82
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Chapter 4
The Legacy of the X- 15
The T-38 had modified speed brakes and a tweaked flight control system that allowed it to fly the steep approaches
used by the orbiter.
Dennis R. Jenkins, Lockheed SR-71/YF-12 Blackbird, (WarbirdTech Series Volume 10, Specialty Press, 1999), p. 37-
38; Dennis R. Jenkins, The History of Developing the National Space Transportation System: The Beginning through
STS-75 (second edition; Cape Canaveral, Florida: Dennis R. Jenkins, 1997), p. 179.
Ronald G. Boston, "Outline of the X-15's Contributions to Aerospace Technology," typescript available in the NASA
Dryden History Office, pp. 16, 20.
J. D. Hunley, "The Significance of the X- 15," 1999, an unpublished typescript available at the DFRC History Office.
Ibid., p. 11.
Ronald G. Boston, "Outline of the X-15's Contributions to Aerospace Technology," typescript available in the NASA
Dryden History Office, p. 17; Burke, Melvin E., X-15 Analog and Digital Inertial Systems Flight Experience, NASA
Technical Note D-4642, 1968, pp. 1-2, 19.
J. D. Hunley, "The Significance of the X-15," 1999, an unpublished typescript available at the DFRC History Office.
Ibid.
Wendell H. StiUwell, X-15 Research Results, (Scientific and Technical Information Branch, NASA, Washington, DC:
NASA SP-60, 1965), pp. 78-79; Milton O. Thompson, At the Edge of Space: the X-15 Flight Program (Washington,
DC: Smithsonian Institution Press, 1992), p. 209.
Milton O. Thompson, At the Edge cf Space: the X-15 Flight Program (Washington, DC: Smithsonian Institution Press,
1992), pp. 87 and 188; Wendell H. Stillwell, X-15 Research Results, (Scientific and Technical Information Branch,
NASA, Washington, DC: NASA SP-60, 1965), p. 79.
Milton O. Thompson, At the Edge of Space: the X-15 Flight Program (Washington, DC: Smithsonian Institution Press,
1992), pp. 188, 210, 263; John V. Becker, "The X-15 Program in Retrospect" (paper presented at the 3rd Eugen Sanger
Memorial Lecture, Bonn, Germany, 4-5 December 1968), p. 6; J. D. Hunley, "The Significance of the X-15," 1999,
an unpublished typescript available at the DFRC History Office.
Ronald G. Boston, "Outline of the X-15's Contributions to Aerospace Technology," typescript available in the NASA
Dryden History Office, p. 20.
Dennis R. Jenkins, Lockheed SR-71/YF-12 Blackbird, (WarbirdTech Series Volume 10, Specialty Press, 1999), p. 37-
38; Jenkins, Dennis R., Space Shuttle: The History of Developing the National Space Transportation System, 1996, p.
224.
Letter from John V. Becker to Dennis R. Jenkins, 10 January 2000.
John V. Becker, A Hindsight Study of the NASA Hypersonic Research Engine Project, unpublished study conducted
under NASA Contract NAS 1-14250, 1 July 1976. Typescript available in the files of the DFRC History Office.
Letter from John V Becker to Dennis R. Jenkins, 10 January 2000.
John V. Becker, A Hindsight Study of the NASA Hypersonic Research Engine Project, unpublished study conducted
under NASA Contract NAS1-14250, 1 July 1976. Typescript available in the files of the DFRC History Office.
J. D. Hunley, "The Significance of the X-15," 1999, an unpublished typescript available at the DFRC History Office.
Letter, William H. Dana, Chief, Flight Crew Branch, DFRC, to Lee Saegesser NASA History Office, transmitting a
copy of the SETP paper for the file, in the files of the NASA History Office. A slightly rewritten (more politically cor-
rect) version of the paper was later published as The X-15 Airplane — Lessons Learned (American Institute of
Aeronautics and Astronautics, a paper prepared for the 31st Aerospace Sciences Meeting, Reno Nevada, AIAA-93-
0309, 11-14 January 1993)
Personal observation of the author, who spent two years (off and on) helping the X-33 program.
Major Michael J.
Adams poses in front
oftheX-15-1.
Major Adams became
the only fatality of the
X-15 program when
he was killed on Flight
#191 while returning
from high altitude.
Adams was posthu-
mously awarded
Astronaut Wings for
his last flight.
(Tony Landis
Collection)
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
83
The Legacy of the X-25
Chapter 4
Technicians at the
Flight Research
Center work on the
XLR99 engine. Note
the corrugations on
the aft end of the
fuselage sponsons
and vertical stabilizer.
This was one of the
keys to allowing the
X-15 to withstand the
high temperatures
encountered during
hypersonic flight. The
blunt ends of the verti-
cals and fuselage tun-
nels alone created as
much drag as experi-
enced by an F-104
fighter. (San Diego
Aerospace Museum
Collection)
A great deal of X-15
research did not
involve the actual air-
craft. Here a rocket
sled is being used to
test the ejection sys-
tem. (Jay Miller
Collection)
84
Hypersonks Before the Shuttle — Monographs in Aerospace History Number 18
Appendix 1
Resolution Adopted by NACA Committee on Aerodynamics, 5 October 1954
Appendix 1
Resolution Adopted by NACA Committee on Aerodynamics, 5 October 1954
This resolution was
the official beginnings
of theX-15 research
airplane program.
RESOLUTION ADOPTED BY NACA
COMMITTEE ON AERODYNAMICS, 5 OCTOBER 1954
WHEREAS, The necessity of maintaining supremacy
in the air continues to place great urgency on solving
the problems of flight with man-carrying aircraft at
greater speeds and extreme altitudes, and
WHEREAS, Propulsion systems are now capable of
propelling such aircraft to speeds and altitudes that
impose entirely new and unexplored aircraft design
problems, and
WHEREAS, It now appears feasible to construct a
research airplane capable of initial exploration of
these problems,
BE EC HEREBY RESOLVED, That the Committee on
Aerodynamics endorses the proposal of the inmediate
initiation of a project to design and construct a
research airplane capable of achieving speeds of the
order of Mach Number 7 and altitudes of several hundred
thousand feet for the exploration of the problems of
stability and control of manned aircraft and aerodynamic
heating in the severe form associated with flight at
extreme speeds and altitudes*
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
85
Signing the Memorandum of Understanding
Appendix 2
Appendix 2
Signing the Memorandum of Understanding
DEPARTMENT OF THE AIR FORCE
WMHMOIDN
NOV 9 1954
MEMORANDUM FOR THE ASSISTANT SECRETARY OF THE NAVY FOR AIR
SUBJECT: Principles for the Conduct of a Joint Project for a New
High Speed Research Airplane
1. The Air Force concurs in the establishment of a joint NACA-
Navy-Air Force project to design and construct a research airplane
capable of achieving speeds of the order of Mach Number 7 and altitudes
of several hundred thousand feet.
2. Attached is a Memorandum of Understanding, signed in tripli-
cate by the Air Force, setting forth the principles for the conduct
by the NACA, the Navy, and the Air Force of this joint project. It
is requested that the Navy sign this Memorandum, in triplicate, and
forward the signed copies to the Director of the NACA for signature
and distribution back to the signatory agencies.
3. The Air Force is designating Brigadier General B. S. Kelsey,
Deputy Director of Research and Development, as the Air Force representa-
tive on the "Research Airplane Committee" referred to in paragraph B
of the Memorandum of Understandi n g ■
(Blgned)
Trevor Gardner
Special Assistant (R&D)
Enclosure
Memo of Understanding
w/1 incl (in trip)
The first of three let-
ters attached to the
Memorandum of
Understanding that
created the X-1 5
research program.
Since it was nominally
an Air Force program,
the Air Force began
the signature process.
The early 1 950s was
an era where carbon
paper and onion-skin
copies were kept.
Forty-five years later
they are not repro-
ducible, so the three
letters have been
recreated.
The letters remained
SECRET until 3 July
1 963 when they were
downgraded to
CONFIDENTIAL.
It was not until 9
November 1966 that
they were finally
declassified.
86
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Appendix 2
Signing the Memorandum of Understanding
The Navy was next to
sign the Memorandum
of Understanding. The
letter is not dated, but
other sources list it as
being sent on 21
December 1954.
oconrr
DEPARTMENT OF THE NAVY
OFFICE OFTHE SECRETARY
WASHINGTON
022421
Dear Doctor Dryden:
The enclosed copy of a Department of the Air Force memorandum of 9 November
1954 signed by Mr. Trevor Gardner, Special Assistant (R & D) expresses the
Air Force concurrence in the establishment of a joint NACA-Navy-Air Force
project to design and construct a research airplane capable of achieving
speeds of the order of Hach Number 7 and altitudes of several hundred
thousand feet. The Department of the Navy also concurs in the establish-
ment of this joint project.
The enclosed Memorandum of Understanding, signed in triplicate by the Navy
and the Air Force, sets forth the principles for the conduct by the NACA,
the Navy and the Air Force of this joint project. This Memorandum of
Understanding is forwarded for signature by the Director of the NACA and
for distribution back to the signatory agencies.
HADM R. S. Hatcher USN, Assistant Chief for Research and Development,
Bureau of Aeronautics, is designated as the Navy representative on the
"Research Airplane Committee" referred to in paragraph B of the Memorandum
of Understanding. The Air Force representative on this committee is desig-
nated in the enclosed Department of the Air Force Memorandum.
Sincerely yours,
(signed)
J.H. Smith, Jr
Assistant Secretary of the Navy (Air)
Dr. Hugh L. Dryden
Director, national Advisory Committee for
Aeronautics
1512 H Street, N.W.
Washington 25, D.C.
Encl:
Copy of Department of the Air Force
Memorandum of 9 Nov 1954
Memorandum of Understanding (in triplicate)
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
87
Signing the Memorandum of Understanding
Appendix 2
NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
WASHINGTON
December 23, 1954
Mr. Trevor Gardner
Special Assistant for Research and Development
Department of the Air Force
4E964 National Defense Building
Washington 25, D.C.
Dear Mr. Gardner:
The National Advisory Committee for Aeronautics con-
curs in the establishment of a joint NACA-Navy-Air Force project
to design and construct a research airplane capable of achieving
speeds of the order of mach Number 7 and altitudes of several
hundred thousand feet.
The Memorandum of Understanding setting forth the
principles for the conduct of this joint project has now been signed,
in triplicate, by the Air Force, Navy, and NACA. A signed copy is
forwarded to you herewith.
The "Research Airplane committee" referred to in
paragraph B of the Memorandum of Understanding is composed of
the following members:
Brigadier General B.S. Kelsey, USAF, Deputy Director,
Research and Development, U.S. Air Force
Rear Admiral R.S. Hatcher, USN, Assistant Chief for
Research and Development, Navy Bureau of Aeronautics
Dr. Hugh L. Dryden, Director, National Advisory Commit-
tee for Aeronautics
Sincerely yours,
(signed)
Hugh L. Dryden
Director
Enc.
HLDbkl
Hugh Dryden, from
NACA, was the final
signature, on the last
working day of 1954.
This set in motion a
chain of events that
would lead to the
design of the fastest
manned aircraft yet
conceived, and the
construction of three
flight research
vehicles.
The first of these
would fly less than five
years later.
88
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Appendix 2
Signing the Memorandum of Understanding
The Memorandum of
Understanding that
set up the "Research
Airplane Committee"
and established the
workings of the
X-15 research
program.
The 5 October 1954
recommendation from
the NACA Committee
on Aeronautics was
attached as a
reference.
GROUP J DOWNGRADF TO:
;._c :ZT ON -
corj-.n^rgriAL on ■ £&.>? ... 7 /*/£3
DCCLASSIFY ON //-^.-^.J
MMEANDOM CF UNDERSTANDING
SUBJECT: Principles for the Conduct by the NACA, Havy and Air Force
of a Joint Project for a New High Speed Research Airplane
A.
B.
C.
D.
E.
F.
G.
A project for a high speed research airplane shall be conducted
jointly by the NACA, .the Navy and the Air Force to implement
the recommendations of the NACA Committee on Aerodynamics, as
adopted on 5 October 1954, copy attached.
Technical direction of the project will be the responsibility of
the Director, NACA, acting with the advise and assistance of a
"Research Airplane Committee" composed of one representative each
from the NACA, Navy and Air Force.
Financing of the design and construction phases of the project
shall be determined jointly by the Havy and Air Force.
Administration of the design and construction phases of the
project shall be performed by the Air Force in accordance with
the technical direction as prescribed in paragraph 3.
The design and construction of the project shall be conducted
through a negotiated contract (with supplemental prime or sub-
contracts) obtained after evaluating competitive proposals
invited from competent industry sources. The basis for solicit-
ing proposals will be the characteristics determined by the
configuration on which the NACA has already done much preliminary
design work.
Upon acceptance of the airplane and its related equipment from
the contractor, it will be turned over to the NACA, who shall
conduct the flight tests and report the results of sane.
The Director, NACA, acting with the advise and assistance of
the Research Airplane Committee, will be responsible for making
periodic progress reports, calling conferences, and disseminating
technical information regarding the progress and results of the
project by other appropriate media subject to the applicable
laws and executive orders for the safeguarding of classified
security information.
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
89
Signing the Memorandum of Understanding
Appendix 2
Msmorandum of Understanding, "Principles fear the Conduct by the NACA,
Navy and Air Force of a Joint Project for a New High Speed Research
Airplane™
H. Accomplishment of this project is a matter of national urgency.
1 Incl
Resolution Adopted by
NACA Committee on
Aerodynamics, 5 Oct 54
Director, NACA
N3ACR
f\j<XVX>
Assistant ZjJ. .lury o" tha Kavy (Air)
C~^
R- wcta.
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Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Appendix 2
Signing the Memorandum of Understanding
RE 30LOTI0H iDOEEED BI NiCA
COMMITTEE ON AERODINAMIDS, 5 OCTOBER 1954
WHEREAS, The necessity of maintaining supremacy
in the air continues to place great urgency on solving
the problems of flight with man-carrying aircraft at
greater speeds and extreme altitudes, and
WHEREAS, Propulsion systems are now capable of
propelling such aircraft to speeds and altitudes that
Impose entirely new and unexplored aircraft design
problems, and
WHEREAS, It now appears feasible to construct a
research airplane capable of initial exploration of
these problems,
BE IT HEREBY RESOLVED, That the Committee on
Aerodynamics endorses the proposal of the immediate
initiation of a project to design and construct a
research airplane capable of achieving speeds of the
order of Mach Number 7 and altitudes of several hundred
thousand feet for the exploration of the problems of
stability and control of manned aircraft and aerodynamic
heating in the severe form associated with flight at
extreme speeds and altitudes.
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
91
Preliminary Outline Specification for High- Altitude, High'Speed Research Airplane
Appendix 3
Appendix 3
Preliminary Outline Specification
CONFIDENTIAL
PRELIMINARY. OUTLINE SPECIFICATION
FOR
HIGH- ALTITUDE, HIGH-SPEED RESEARCH AIRPLANE
October 15, 195^
1. STATEMENT OF PROBLEM AND OBJECTIVE:
la The next major advance in aircraft performance will plunge the
aircraft designer into a speed range where the accompanying temper-
ature effects would cripple the strength of conventional aircraft
materials and structures and into an altitude range where the air
pressure is too low for conventional aerodynamic controls. In
addition, certain physiological and environmental problems asso-
ciated with the pilot of such a high-speed high-altitude airplane
are anticipated. Many of the most important problems in this field
can be satisfactorily investigated only with a manned full-scale
flight vehicle.
lb In order to provide the fundamental research information essential
to the practical solution of these problems in this country, a
need exists for a research airplane capable of exploring the
speed and altitude regimes in which these problems are encountered .
lc As the need for the exploratory data is acute because of the rapid
advance of the performance of service aircraft, the minimum
practical and reliable airplane is required in order that the
development and construction time, be kept to a minimum.
2. APPLICABLE SPECIFICATIONS, STANDARDS, DRAWINGS, AND OTHER PUBLI-
CATIONS:
2.1a General specifications for the design and construction of airplanes
for the United States Navy, SD-2^G dated shall be fol-
lowed where applicable .
2.1b Materials, process, design and installation specifications and
equipment drawings, applicable to piloted aircraft in effect as of
this date shall be followed where applicable.
2.1c Deviations from applicable Government specifications and standards
will be encouraged provided these deviations are directed toward
the accomplishment of the objective set forth in paragraph lc.
2.2a Specifications and standards shall be used in the order of precedence
set forth in ANA Bulletin Xk^c.
CONFIDENTIAL
The preliminary speci-
fication for the future
X-15 did not differ
substantially from the
final version published
a few weeks later.
Although engines
were not specifically
discussed in the writ-
ten document, the air-
craft depicted in
Figure 2 was powered
by three Hermes A3A
engines and assumed
a launch by a modified
B-50 carrier aircraft.
92
Hypersohics Before the Shuttle — Monographs in Aerospace History Number 18
Appendix 3
Preliminary Outline Specification for High- Altitude, High-Speed Research Airplane
CONFIDENTIAL™-
3-
REQUIREMENTS:
3.1a GENERAL - Major considerations that established the required
performance of the airplane are:
(a) For exploring the aerodynamic heating problem, the structure
must be subjected to extreme heating conditions . Allowable
skin temperatures are of the order of 1200° F and maximum
heating rates of the order of thirty (30) BTU/sq. ft/sec.
are desired. Altitude-speed requirements are also such that
radiation heat loss is of comparable magnitude to the con-
vective heat input with resultant skin temperatures well
below adiabatic boundary- layer temperature .
(b) For exploring the stability and control problems of a manned
high-altitude aircraft, the speed-altitude capabilities of the
research airplane should permit the establishment of flight
conditions for which aerodynamic forces are negligible com-
pared with inertia forces, thus requiring the use of auxiliary
controls .
(c) For exploring physiological factors affecting pilot response,
the research airplane should be capable of effecting periods of
"weightlessness" for a long enough period to permit exploration
of this field.
(d) Provisions should be made to substitute an observer in the
space alloted for research instrumentation.
3-2a PERFORMANCE
(a) The airplane shall be capable of achieving a speed of at
least 6600 ft/sec.
(b) The airplane shall be capable of attaining an altitude of at
least 250,000 feet.
3-2b AIRPLANE WEIGHT AND SIZE - The size and weight of the airplane shall
be such as to permit air launching from a mother airplane; such as
the B-50, B-36, or B-52, thus effectively providing a two-stage
vehicle .
3-3a
OPERATIONAL FACTORS - The research airplane will normally be operated
from and in the vicinity of the Edwards Air Force Base, California.
The presence of the large landing areas afforded by the dry lakes in
the vicinity may be taken into consideration in the design of the
airplane for the landing phase of the flights .
CONFIDENTIAL
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
93
Preliminary Outline Specification for High- Altitude, High-Speed Research Airplane
Appendix 3
CONFIDENTIAL
3.ta
3.1rt>
k.
k.Xa.
k.Xb
ij.lc
U.ld
k.le
5-
5-la
5.1b
- 5
VISION - A reasonable degree of vision, direct or not, should be
afforded the pilot particularly in the landing approach attitude .
Vision for the observer shall be provided only to the extent that
scientific observation may be satisfied.
STABILITY AND CONTROL:
The flying qualities and general handling characteristics of the
airplane during all phases of the flight, both inside and outside
of the atmosphere shall be adequate to permit satisfactory ful-
fillment of the mission and utilization as specified herein.
The wing and tail arrangement shall be such as to offer promise
in the light of existing aerodynamic knowledge, of attaining
good stability and control characteristics throughout angle-of-
attack range at low speeds, as well as at high speeds .
Controls shall be provided to permit changing airplane attitude
in the absence of aerodynamic forces .
Where an artificial feel system is employed, the system shall be
foolproof, reliable, and as simple as possible consistent with
the force requirements . Any complicated and/or apparently
unreliable system shall be unacceptable .
Through combination of aerodynamic features, such as powerful
dive brakes and/or large drag at high angles of attack, and
structural features, such as thick skin, auxiliary cooling, and/or
high temperature alloys, it shall be possible to recover from
flights to maximum speed or maximum altitude without exceeding
the allowable temperature limits for the structure or the accel-
erations currently encountered in combat with fighters .
STRUCTURAL DESIGN CRITERIA:
The high temperatures and aerodynamic heating loads anticipated
in the operating regime of the airplane require that careful
attention be given to the choice of structural materials and/or to
methods for cooling and/or insulating the surfaces .
The design normal loads shall be
equivalent .
+7.50g and -Jg or the A.F. fighter
CONFIDENTIAL
94
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Appendix 3
Preliminary Outline Specification far High' Altitude, High-Speed Research Airplane
CONFIDENTIAL
6.
6.1a
6.1b
6.1c
6. Id
6.2a
6.3a
7-
7 -la
8.
8.1a
- h
FURNISHINGS AND EQUIPMENT:
Provisions for cockpit pressurization and air conditioning shall
be adequate for flights to maximum speed and altitude.
Provisions shall be made to permit the use by the pilot of full
pressure suit . The pilot shall have reasonable protection from
radiated and conducted heat.
Suitable escape provisions shall be provided for the pilot.
Provisions for breathing oxygen shall be sufficient for the complete
flight.
The observer shall be provided with protection and escape provisions
equal to those provided for the pilot.
All instruments necessary for the proper performance of the airplane
shall be provided for the pilot.
PROPULSION:
The propulsion system chosen shall be suitable for a manned aircraft.
A list of powerplants which with reasonable development may be used
for the project follows:
(List to be provided by BuAer and WADC)
RESEARCH INSTRUMENTATION:
A weight allowance of 500 pounds and a volume allowance of 5 cubic
feet shall be provided for research instrumentation. Provision
for pressurization and cooling must also be made. Thermocouples
shall be installed for determining the temperature distribution
throughout the airframe.
CONFIDENTIAL
Monographs in Aerospace History Number 18 — Hypersomics Before the Shuttle
95
Preliminary Outline Specification for High- Altitude, High-Speed, Research Airplane
Appendix 3
jCONFIDENTIAL
APPENDIX
USE OF NACA FACILITIES FOR FINAL DEVELOPMENT:
High Mach number wind tunnel and structural development work are
essential to establish the final design of such a research airplane.
Facilities for such work are in existence at NACA Laboratories and will
be made available for development of the selected design.
SUGGESTED MEANS OF MEETING GENERAL REQUIREMENTS:
The NACA has made studies to determine if, on basis of the existing
knowledge, it would be possible to develop and construct an airplane
capable of meeting the preceding requirements. A typical flight plan
is on figure 1. The airplane configuration evolved is shown in figure 2.
Figure 1 illustrates one of the flight trajectories that is possible
with the airplane within the temperature limits of the structure. The
airplane is launched from the mother ship at 35*000 feet. Burnout occurs
at an altitude of 11+6,000 feet and at a speed of 6600 feet per second.
In its subsequent ballistic trajectory, an altitude of 280,000 feet is.
achieved and for about 130 seconds in this trajectory the dynamic pressure
is less than 6 pounds per squaiefoot. During this period of time, the
pilot will be required to change the attitude of the airplane from nose-up
to nose-down as required for reentry using nonaerodynamic controls. In
the reentry portion of the trajectory, the combined use of dive brakes
and moderate lift on the airplane may be used to avoid excessive skin
temperatures .
Figures 3 and k show schematically an internal wing structure which
would permit thermal expansion of the wing without the production of large
thermal stresses. Some of the more important features are noted in
figure 2 or given below:
(a) Size and weight are such as to permit use of a B-50 mother
ship for launching.
(b) Wing and tail arrangement offering promise of attaining
good stability and control characteristics throughout angle-of -attack
range at low speeds as well as at high speeds
(c) Split tail surfaces affording powerful means for providing
required stability at very high speeds and avoiding the necessity
for excessively large stabilizing surfaces
CONFIDENTIAL
96
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Appendix 3
Preliminary Outline Specification for High- Altitude, High'Speed Research Airplane
CONFIDENTIAL
(d) Split flaps on wing and tail surfaces to provide powerful
dive brakes. Fuselage dive brakes may also be necessary.
(e) Rounded leading edge and leading-edge sweep of wing and
tail surfaces, greatly reducing rate of heat transfer into these
surfaces
(f) Skin thickness of 0.1-inch Inconel alloy, providing adequate
heat sink to accomplish desired flight trajectories without exceeding
a 1200° F temperature limit
(g) An interior web and rib detail minimizing the thermal stress
problem by permitting free expansion of wing elements
(h) Use of skid type landing gear to avoid tire cooling problems
CONFIDENTIAL
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
97
600 800
Range, thousands of feet.
1200
Figure 1.- atypical flight trajectory.
9
f
c I
o
- 130 seconds (approx.)-
M = 6.25
q - 6.8 psf
v - 6250 fps
* Burnout
v - 6600 fps
< 1
( )
t <
II
I
a.
W
o
3
3
>
I
3/4-inch Rad.
03
CO
r
8-
C )
1
27. 4 ft
< )
c >
u
t )
5 -feet dia.
Thrust , . „
= ,, . . . (sea level), l.o
Gross Weight v ''
Fuel Weight ,
Gross Weight
Spec . Impulse (Ale . -Lox . ) , 223 sec .
V (B-50 Launch), 6800 ft/sec
Gross Weight
Fuel »
Wing Loading
Aspect Ratio
Thrust
30,000 lb
18,000 lb
48 PSF (empty)
3.0
54,000 lb (sea level)
(3 Hermes A3A Engines)
Figure 2.- Suggested configuration for research airplane.
s
8
§•
X
I
ST
X
pa
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5"
1
t
o
s
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■3-"
Surveying the Dry lakes
Appendix 4
Appendix 4
Surveying the Dry Lakes
TO
FROM
PHONE
SUBJECT
NORTH AMERICAN AVIATION, INC.
INTER-OFFICE LETTERS ONLY
6.R. Mellinger DEPARTMENT 56
G. P. Lodge DEPARTMENT 56
DATE 1 December X959
Survey of Dry lakes in California. Nevada and Utah
A Burvey was made of approximately 50 dry lakes in California, Nevada and Utah
area to ascertain which lakes would be suitable for emergency landing sites for
the X-15 airplane.
The method used for determining surface hardness was dropping an IB pound, 5 inch
diameter steel ball from a 6 foot height and measuring the diameter of the imprint
in the surface. A diameter of less than 3 1/4 inches is considered satisfactory.
In addition to the steel ball check, a 3/8 inch and a 1/2 inch diameter blunt end
steel rod was also used to probe the surface to determine the thickness of the
crust and soil condition under the crust which would have a direct effect upon
load bearing qualities of the lake. A force of 200 pounds was applied to the rods
and the depth of penetration measured.
Listed below are the lakes investigated and comments regarding the condition and use:
Location - 35° 17 N, 117° 28 W
Cuddeback Lake: Surface crust is moderately rough and damp in spots. Steel ball
imprint varies from 3 to 4* . 3/8 rod penetration up to 12* . 1/2 rod 2 to 10" .
This lake is considered marginal for emergency recovery.
Location - 35° 44 N, 117° 30 W
Searles lake: No landing made. Surface appeared soft and wet. Water and diggings
on lake bed.
Not considered usable.
Location - 36° 00 N, 117° 14 W
Ballarat Lake: No landing made. Surface appeared soft and sandy with wet spots.
One small area of the lake bed had checked surface. Road crosses north end.
Not considered usable.
Location - 36° 20 N, 117° 25 W
Panamint Springs: No landing made. Surface at south end was drifted sand. North
end appeared hard but too small for X-15 use. A few ditches are on lake bed. Paved
road crosses the north end.
Not considered usable for X-15.
A major task that
needed completed
before the first X-15
flight was a survey of
available emergency
and contingency land-
ing areas along the
projected flight corri-
dor. Since the X-15
was equipped with
skid-type landing gear,
the only acceptable
landing areas were
dry lakebeds.
North American and
the Air Force made
several trips to survey
the dry lakes along
the flight corridor and
to make tests on the
most promising. The
lakebed had to be
smooth, long enough,
and hard enough to
accommodate the
X-15.
102
Hypersonics Before the Shuttle — Monographs in Aerospace. History Number 18
Appendix 4
Surveying the Dry lakes
6. R. Mellinger from G. P. Lodge
Page two
1 December 1959
Location - 36" 30 N, 116° 55 W
Death Valley: No landings made. No usable spots noted.
Location - 36" 30 N, 116° 32 W
Scranton: No landings made. Hater on surface.
Not considered usable.
Location - 36° 15 N, 116° 23 W
Death Valley Junction: No landing made. Surface appeared soft and numerous diggings
on lake bed.
Not considered usable.
Location - 36° 13 N. 116° 10 W
Stewart Valley: No landing made. Surface appeared soft and had drifted sand.
Not considered usable.
Location - 36° 17 N. 116° 03 H
Pahrump: No landing made. Surface appeared soft with drifted sand and brush
growing.
Not considered usable.
Location - 36° 00 N, 115° 57 W
Hidden Hills: Elevation 2,000 feet. Surface hard and smooth. Steel ball imprint
3 to 3 1/2 inches. 3/8 Rod penetration 3 1/4 to 3 1/2 inches. Hater draining in
at north end. Approximately 15,000 feet of usable lake on 150° - 330° headings.
Approximately 2,000 to 3,000 feet more length available when dry. Access from
paved road 10 miles east.
This lake considered good for emergency recovery.
Location - 35° 43 N, 115° 35 W
Hesguite Lake: No landing made. Surface appeared soft with water and brush on
lake bed.
Not considered usable.
Location - 35° 32 N, 115° 22 H
Ivanpah Lake: Elevation 3,000 feet. Large lake with smooth moderately hard surface.
Steel ball imprint 3 1.4 to 3 1/2 inches. 3/8 Rod penetration 10 to 12 inches, 1/2
Rod 1 to 7 inches. Soil under crust was damp. Paved road crosses north end of lake
bed. Approximately 23,500 feet usable length on 160° - 340° headings.
This lake is considered marginal for emergency recovery.
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
103
Surveying the Dry lakes
Appendix 4
G. R. Mellinger from 6. P. Lodge
Page three
1 December 1959
Location - 35° 40 N, 115° 22 W
Reach lakes Smooth, slightly cracked surface. Steel ball imprint 3 1/2 to 4 inches.
3/8 Rod penetration 15 inches. Crust breaks up easily. Soil loose under 1 inch
crust. Railroad track crosses lake.
Not considered favorably for emergency recovery.
Location - 36° 27 M, 114.° 52 W
Dry Lake: No landing made. Surface too small for X-15 use.
Not considered usable for X-15.
Location - 36° 58 N, 115° 15 W
Cabin Springs: No landing made. Surface rough and uneven. Brush on lake bed
around edges.
Not considered usable.
Location - 37° 20 N, 114° 55 W
Delamar Vallay: Elevation 4,000 feet. Surface moderately hard and smooth. Dry
and hard under surface. Steel ball imprint 3 inches. 3/8 Rod penetration 2 to
2 1/2 inches. Usable length 13,500 feet on 0° - 180° headings. Power line on S.E.
corner. Wind was 10 - 15 raph from North. Access to lake is from Alamo on U.S. 93.
This lake is considered good for emergency recovery.
Location - 37° 45 N, 114° 49 W
Dry Lake Valley: No landing made. Surface appeared soft with drifted sand.
Not considered usable.
Location - 37° 55 N, 115° 20 W
Coal Valley: No landing made. Surface appeared to be soft sand.
Not considered usable.
Location - 38° 31 N. 115° 37 W
Currant Lake: Surface rough and soft. Shallow ditcheB across center of lake bed.
Not considered usable.
Location - 39° 17 N, 115° 15 W
Jakes Lake: Large lake. Surface soft and rough with shallow ditches. Grass growing
on lake bed and cattle grazing.
Not considered usable.
104
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Appendix 4
Surveying the Dry lakes
G. R. Hellinger from 6. P. Lodge
Page four
1 December 1959
Location - 39° 40 N. 115° 43 W
Newark Valley: Long lake. Surface smooth and soft. Steel ball imprint 4 1/2 inches.
Crust breaks up easily. Soil under crust damp.
Not considered usable.
Location - 40° 00 N. 115° 58 W
Diamond Lake: No landing made,
size of Rogers.
Not considered usable.
Location - 40° 23 N. 115° 25 W
Franklin Lake: Large lake partially covered with grass.
soil under soft crust. Steel ball imprint 4 1/2 inches,
inches .
Surface smooth and soft. Lake bed approximately
Surface soft with loose
3/8 Rod penetration 12
Not considered usable.
Location - 40° 08 N. 114° 42 W
Coshute Lake: No landing made. Surface soft, cattle on lake bed.
Not considered usable.
Location - 41° 05 N, 113° 55 W
Area immediately east of Pilot Peak, surface white, smooth and soft. Steel ball
imprint 4 1/2 inches. 1/2 Rod penetration 12 inches plus. Soil under white salt
film wet.
Not considered usable.
Location - 40 ° 46 N, 113° 50 W
Bonneville Flat Race Track: Large area with white surface. Long black line on
headings of 30° - 210° marks course. Surface adjacent to line (1/4 to 1/2 mile
each side) exceptionally hard and composed of salt. Steel ball imprint 1 3/4 inches.
3/8 Rod penetration zero. Darker colored areas to each side soft. The area adjacent
to and parallel with the race track is considered an excellent emergency recovery
site.
Location - 40° 45
114° 42
No landing made. Usable surface too small.
Not considered usable.
Location - 40° 47 N, 114 ° 57 W
Snow Hater Lake: No landings made. Surface soft with water on west portion.
Not considered usable.
Monographs in Aerospace History Number 18 — Hypersonks Before the Shuttle
105
Surveying the Dry lakes
Appendix 4
G. R. Mellinger from G. P. Lodge
Page five
1 December 1959
Location - 40" 00 N, 116" 40 W
(Haiti Hot Spring): Large lake; surface smooth, dry and soft. Steel ball imprint
4 to 4 1/2 inches. 3/8 Rod penetrated full length. Crust thin and breaks up easily.
Soil under crust dry and powdery.
Not considered usable.
Location - 40° 11 N. 116" 50 W
No name: No landing made. Surface appeared soft.
Not considered usable.
Location - 40° 25 N. 117° 20 W
No name: No landing made. Surface appeared soft. Cattle on lake bed had left deep
tracks. Top of cinder cone crater above surface at south end.
Not considered usable.
location - 40' 14 M, 117° 58 W
Buena Vista Valley: Large lake. No landing made. Surface appeared soft, drifted
sand and deep cattle and car tracks on all portions of lake bed.
Not considered usable.
Location - 39° 20 N, 118° 30 W
Carson Sink: Vary large area. North portion covered with wide shallow ditches and
drifted sand. Landing area approximately 2 miles S.W. of target cone in N.E. portion
of lake. Surface smooth, checked and soft. Steel ball imprint 4 to 4 1/2 inches.
3/8 Rod penetration full length. Soil under crust damp and loose. Would not pack.
Touch and go landings made on other portions of lake bed indicated soft surface on
entire lake.
Not considered usable.
Location - 39° 20 N. 119° 25 W
No name: No landing made. Small lake with sandy brush covered surface.
Not considered usable.
Location - 39° 18 N, 119° 04 W
No name: No landing made. Brush covered surface.
Not considered usable.
Location - 39° 23 N. 118° 53 W
No name: Two lakes. No landing made. Surface appeared soft and brush covered.
Not considered usable.
106
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Appendix 4
Surveying the Dry lakes
G. R. Mellinger from Q. P. Lodge
Page six
1 December 1959
Location - 39° 09 N. 118° 42 W
No name: No landing made. Surface appeared soft and covered with ditches.
Not considered usable.
Location - 39° 22 N. 118° 36 W
No name: Small lake. No landing made. Surface appeared soft.
Not considered usable.
Location - 39° 19 N. 118° 30 W
No name: Large lake. No landing made. Surface appeared soft. Ditches on north
end of lake bed.
Not considered usable.
Location - 39° 16 N, 118° 16 W
Labou Flat: Small lake. No landing made. Road crosses surface and gunnery targets
installed on east side.
Not considered usable.
Location - 39° 37 N, 117° 39 W
No name: large lake. Elevation 5,000 feet. Mountains on east, north and west
sides. Wide valley to south. Lake bed is approximately 7 miles long on 30° - 210°
headings and 3 to 4 miles wide. Surface is smooth and moderately hard. Steel ball
imprint varied from 3 to 3 1/2 inches. 3/8 Rod penetration varied from 10 to 12
inches at north end and center to 1 to 3 inches in light colored area at south end.
Dark colored area at south end is soft. Boil 5 to 6 inches below crust dump. Best
touch-down point would be at south end in light colored area on 30° heading. Access
via dirt road from Eastgate, Nevada.
This lake is considered favorably for emergency recovery.
Location 39° 20 N. 117 ° 29 W
Smiths Ranch: Large lake with smooth hard surface. Elevation 5700 feet. Lake
bed is 7 to 8 miles long. Surface at south end is rougher but harder than north
end. Roughness is result of wider cracks that existed at one time in surface.
Steel ball imprint at north end 3 to 3 1/4 inches, center 2 3/4 to 3 inches, south
end 2 1/4 to 2 1/2 inches. 3/8 Rod penetration varied front 1 inch at north end,
2 1/2 or 3 1/2 inches at center to 1/2 or 3/4 inches at south end. Crust thickness
varied from 4 to 7 inches. Access is from paved road, U.S. 50 that runs adjacent to
lake bed.
This lake is considered an excellent emergency recovery site.
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
107
Surveying the Dry lakes
Appendix 4
G. R. Hellinger from G. P. Lodge
Page seven
1 December 1959
Location - 38" 54 M, 118° 15 W
Gabbs Valley: No landing made. Surface appeared soft, dark and wet.
Not considered usable.
Location - 38" 05 N, 117° 58 W
Columbus Salt Harsh: No landing made. Surface appeared soft and rough with ditches.
Not considered usable.
Location - 38° 01 N. 117° 38 W
Big Smoky Valley: Long narrow lake. Surface rough and uneven. Soft spots on
south half. Steel ball imprint at north end 3 1/4 to 3 1/2 inches. 3/8 Rod penetra-
tion full length. Crust thin and crumbles easily. Soil under crust loose. Approxi-
mately 10,000 to 15,000 feet of surface on headings of 20° - 200° at north end of
lake may be considered satisfactory for jet A/C.
Not considered usable for x-15.
Location 37° 52 N, 117° 23 W
Alkali Springs: Circular shaped lake. Surface smooth, cracked and hard. Elevation
4500 feet. Steel ball imprint 2 1/2 to 2 3.4 inches. 3/8 Rod penetration 1/2 inch.
Crust 4 inches thick. Soil under crust loose. Usable length 9,000 feet on 50° - 230°
heading.
This lake is considered excellent for emergency recovery of jet A/C but too small
for X-15.
Location - 37° 52 N. 117° 04 W
Mud Lake: Circular shaped lake. Elevation 5,000 feet. Surface smooth and hard.
Harked runways exist on headings of 60° - 240° and 170° - 350°. Steel ball imprint
varied from 2 to 3 inches. 3/8 Sod penetration was 1/8 to 1/4 inches except on
east side of lake where it could be pushed in all the way. The east portion of lake
is the softest part. Usable length of surface is 4 to 5 miles in any direction. It is
recommended that touch down not be made on east portion if possible. Access is by
dirt road approximately 10 miles from paved road.
This lake is considered usable for X-15.
Location - 37° 40 N. 117° 41 W
Clayton Valley: No landing made. Area covered with sand dunes.
Not usable.
Location - 37° 26 N, 117° 09 W
No name: No landing made. Surface covered with sand and brush except for small open
area .
Not considered usable
108
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Appendix 4
Surveying the Dry lakes
Q. R. Mellinger from G. P. Lodge
Page eight
1 December 1959
Location - 37" 13 N, 117° 05; W
No name: No landing made. Surface covered with volcanic mud flow. Major portion
of surface wet.
Not considered usable.
Location - 37° 10 N. 117° 10 W
No name: Small lake. Surface smooth and hard. Steel ball imprint 2 1/2 to 2 3/4
inches. 3/8 Rod penetration 2 to 3 inches. Usable length 10,000 feet on 10° - 190°
heading. Numerous small brush covered islands scattered on surface.
This lake is considered usable for emergency recovery of jet A/C but too small for
x-15.
The attached sketch shows the location of the lakes inspected by latitude and longi-
tude. Also included on the sketch are lake beds previously inspected by the writer
and those inspected by L/Col. Anderson and Major White of Edwards Flight Test Center.
The lake beds designated "most usable" were selected from the standpoint of size,
surface conditions and access for recovery of vehicle.
6. P. Lodge
Flight Safety Specialist
GPLilr
Dc: Ferren, Crossfield, White, Roberts, Wilkerson, Helgeson, Cokeley, Harvey,
Richter, Stacey, Beach, Jelinek, O Conner, Lodge, File (10) .
Monographs in Aerospace History Number 18 — Hypersonics Before the. Shuttle
109
Surveying the Dry lakes
Appendix 4
E3REN0
LONG
119°
US-
ROGERS (
117°
116°
® SMITHS
X g B TONOPAH
<3> ©MUD
X
X
X
/
BEATTY0
T7
X v/
* /.
V o
/
X. X
x X
x e
115°
114°
♦ V
/
/
WENDOVER
X X
[T ©BONNEVILLE
f
r /
/
/
Rely
Qlas VEGAS
■41°-
■40'
.39'
■ 38'
.37'
.36'
■ 35'
. 34'
© MOST USABLE
O OKFORX-15
<!> OKFORJETA/C
V MARGINAL
X NOT CONSIDERED USABLE
R TOWN
SURVEY OF
DRY LAKES
This recreation of the
original sketch shows
the location of the
lakes inspected by
latitude and longitude.
Also included on the
sketch are lake beds
previously inspected
by Mr. Lodge and
those inspected by
Lieutenant Colonel
Anderson and Major
White of the Air Force
Flight Test Center at
Edwards AFB.
The lake beds desig-
nated "most usable"
were selected from
the standpoint of size,
surface conditions and
access for recovery of
vehicle.
110
Hypertonics Before the Shuttle — Monographs in Aerospace History Number 18
Appendix 5
R&D Project Card— Project 1226, X-15 Research Aircraft
Appendix 5
R&D Project Card— Project 1226, X-15 Research Aircraft
The Air Force's Air
Research and
Development
Command (ARDC)
was the lead organi-
zation for the develop-
ment and procure-
ment of the
X-15 airplanes. This
"Project Card" initiated
the paperwork for the
project. Like much of
the early
X-15 data, it was clas-
sified SECRET.
SECURITY CLASSIFICATION
RftD PROJECT CARD
TYPE OF REPORT
Mew Project
1. PROJECT TITLE
(CONFIDENTIAL) X-15 Besearch Aircraft
«. BASIC FIELD OK SUBJECT
Technical Development
a. COON I Z ANT AGENCY
ARDC
0. DIRECTING AGENCY
Fighter Aircraft Division, WADC
»A. OFFICE SYMMl
WCSFF
10. REQUESTING AGENCY
NACA - Hq USAF
II. EXTENSION
39159
i. PARTICIPATION. COORDINATION. INTEREST
US Navy (P)
NACA (P)
1. SECURITY it
PROJECT
SECRET
1-1226
taroxr cannot, snoot,
DD-RDBfraftti8
3 ■ PROJECT NUMBER
1226
5. REPORT DATE
7 March 1955
7. SUBFIELD OR SUBJECT SUBGROUP
Ol-Aircraft and Design Studies
12. CONTRACTOR AND/OR LABORATORY
Contractor to be selected
after a Design Competition
This material f*w*irns in'orraa-
tion attocttnji the.natiohai defou*.
ot tks L'h-ev. ,c^a»es fttthfo tin.
moar.inir 'rt t'A- Sk:»ian»« La*s»
Titla :<:. '.'.■..; • -i^rts 7»jr
MJ 1)1. .;,, .■-:•„ .:-,:„ i r
rtv-ijatfcr. c: w-a.-..; : i n .^y
mancap •_ : u a**i&o:iw l ljH«»»ti
In miliirr '
IB. RELATED PROJECTS
X-1A, X-1B, X-1E, X-2
14. DATE APPROVED
7 March 1955
IS. PRIORITY
1-B
Category A-l
TA. TECH OBJ
SR-lf
HA. CONTRACT- W.O. NO.
reb. Continuing
bey. Dee 195o
TOTHar I>9( Phase II,
"See
FISCAL ISTS <M$)
Item 21dC2f
20 REQUIREMENT AND/OR JUSTIFICATION
As a result of studies made by the NACA between June 1952 and July 195U, it was
concluded that the two most serious problems which will be encountered in
flight at very high speeds and altitudes are: (l) prevention of the destruction
of the aircraft structure by the direct or indirect effects of aerodynamic
heating and, (2) achievement of satisfactory stability and control. A review
of existing and planned facilities suitable for these investigations indicates
that while certain phases of these problems can be studied in the laboratory,
there will remain many questions which can only be answered by full scale
flight research. A manned airplane was considered to be feasible since the
nature of the stability and control problem will dictate that the high
performance be attained by moderate incremental increase starting from speeds
and altitudes at which information is already available. Technical Program
, Requirement No. 1-1 dated 6 October 195b was established by Hq USAF with the
ni) FORM A 1 3 PREVIOUS EDITIONS
I JAN 52 OF THIS FORM
MAY BE USED
IRITV CLASJIFICAJJOI
PASE X
OF [i PASES
C5-3cU26
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
111
R&D Project Card— Project 1226, X-15 Research Aircraft
Appendix 5
MO PROJECT CARD ,
COHTIHUATION, SHEET
EECRhT „
SECUR I TY gASSIFIgWrON
SECRET
X-15 Research Aircraft
I. PMJECT TITLE
(CONFIDENTIAL)
2. SECURITY OF
PROJECT
SECRET
1-1226
3. PROJECT NUMBER
1226
». REPORT »n-
7 March 1955
20. (contd)
objective of initiating a new manned research airplane project generally in
accordance with the NACA Secret report, subject, "NACA Views Concerning a New
Research Aircraft," dated August 195k- (Conf)
21. a. Brief
This project is being initiated to develop an air launched, rocket
propelled, manned aircraft capable of flight at speeds of at least 6600 ft/sec
and altitudes of at least 250,000 ft. It has been generally accepted that the
state-of-the-art will support the development of a vehicle capable of this
performance in the 1955-1958 time period. Since the return from this type of
research vehicle diminshes with time, the project will be aimed toward obtaining
a vehicle, not necessarily optimum, which meets the performance requirements
and which will be available for the research program in 3§ years. (Secret)
21. b. Approach
All major airframe contractors have been invited to propose designs in
a competition announced 30 December 1951t« The deadline for the submittal
of proposals is 9 May 1955. The proposals will be evaluated and a recommended
technical order of merit will be established. The recommendation, along with
other pertinent information, will be presented to the "Research Airplane
Committee" for the selection of the design which will be developed. The design
aooroach which has been selected will be presented to the Coordinating Committee
on Piloted Aircraft, Department of Defense, for review and approval. (Conf)
21. c. Subtasks
A task or a project, as required, will be established to develop one of
the four rocket engines being considered to a configuration suitable for this
application. The time available for this task is less than three years and is
considered to be critically short. The rocket engine program will be subjected
to a review immediately upon the selection of the winning airframe design to
determine the availability of the engine in the required configuration. (Conf)
21. d. Other Information
(1) General
The project will be conducted under the guidance of a "Research
Airplane Committee" composed of one representative each from the NACA,
Navy and Air Force. The aipplane will be demonstrated by the
DO pom 613-1
"" i feb ss ° ,a '
PREVIOUS EDITIONS
OF THIS ram MAY
BE USED.
2 OF 1; PAGE
C5--38U26
112
Hypersonics Before the. Shuttle — Monographs in Aerospace History Number 18
Appendix 5
R&D Project Card— Project 1226, X-J5 Research Aircraft
MD PROJECT C*R0_
CONTINUATION SHEET
I. PROJECT TITLE
SECRET
(CONFIDENTIAL) X-l$ Research Aireraft
2. SECURITY OP
PROJECT
SECRET
1-1226
3. PROJECT NUMBER
1226
%. REPORT 0»TF
7 March 1955
21.d.(l)(oontd)
contractor up to Mach 2.0 at moderate altitude after which the Air
Force will conduct a limited Phase II flight test program. After
acceptance of the airplane by the Air Force, it will be placed on
indefinite loan to the NACA for the flight research program. (Conf)
(2) Funds
It is planned to contract for a program which includes mock-up,
static test and three flight articles. The cost of this program
is estimated at a minimum of $2$, COOM, including engine development
costs, over a period of four fiscal years. The Navy is expected
to provide one fourth of the total funds required. Funding for any
one fiscal year will not exceed |10,000M. A breakdown of R&D funds
by fiscal year and an estimate of the man hours required are as
follows:
P600
Manhours
10,000
FT 56
$10,OOOM
U,ooo
FT 57
$8,000M
3,000
FT 58
$ll,000M
3,000
FT 59
$3,000M
7,000
(Conf)
(3) Resource Requirements
(a) A B-36, B-50 or an airplane of comparable size will be required
for modification to the carrier configuration.
(b) All flights of this airplane will be planned for termination
at the AFFTC, although on some flights, the airplane maybe
launched as far away as Salt Lake, Utah.
(c) Additional instrumentation located remotely from Edwards AFB
will be required to monitor and control the flights where remote
launching is required. The nature of this instrumentation and
its location will be established in the course of the development
of the airplane. The requirement for this equipment will probably
not occur until 1959, after the initiation of the NACA fli^it
research program. (Conf)
21. e. Background History
The conception of this airplane seems to have occurred in June I?52 when
the Committee on Aerodynamics of the NACA recommended that the NACA increase its
research on problems of manned and unmanned flight at altitudes between 12 and
50 miles and at Mach Numbers between h and 10. Through a series of studies over
DD
ram 613-1 previous edition*
«■ •» OF THIS FORM NAY
3 OF 1; '»
C5-38U26
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
113
R<S?D Project Card— Project 1226, X-15 Research Aircraft
Appendix 5
R40 PROJECT CA.RD
CONTINUATION SHEET
SLCHET
1. PROJECT TITLE
3i i c*er
(CONFIDENTIAL) X-15 Research Aircraft
2. SECURITY OF
PROJECT
SbCKET
1-1226
3- PROJECT DUMBER
1226
5. REPORT n**'
7 Ma^sh 1955
21.e.(contd)
a two year period conducted independently by NACA's Langley and Ames Labora-
tories and High-Speed Flight Station, NACA concluded that a new research air-
plane capable of exploring that flight regime is both necessary and feasible.
In May 1°5U } NACA proposed to the Air Force that a meeting be held to discuss
the need for a new research airplane. Concurrent with the NACA consideration
of the need for a new research airplane, the Aircraft Panel of the Air Force's
Scientific Advisory Board had also been considering the matter and had formally
recommended that the Air Force initiate action on such a program. A series of
meetings among various elements of NACA, Navy, USA? and Department of Defense
resulted In a decision by the Department of Defense that an Air Force managed
project under the guidance of a joint NACA, Navy, Air Force Steering Committee
would be appropriate. On 6 October 195U, Hq USAF issued Technical Program
Requirement No. 1-1 directing the initiation of the project. (Secret)
21. f. Future plans for the development of other Research Airplanes will be
contingent upon the results of the X-1A, X-1B, X-1E and X-2 flight programs
and the establishment of the need for data in some yet unexplored regime of
flight. (Unci)
21. g. References
(1) NACA Report, subject, "NACA Views Concerning a New Research Airplane,"
dated August l Q £Lu
(2) Hq USAF Technical Program Requirement 1-1 dated 6 October 195U
(3) Hq ARDC Technical Requirement 5ii dated 26 October 195k. (Unci)
21. h. Capt. C.E. McCollough, Jr. WCSFF, 3°lS°
TUs document li
•eenduM uSth
'jm^rMz
DO form 6I3-| PREVIOUS EDITIONS
I FEi ss 0F m|t „„, m,
RE USED.
SECURITY CLASSIFICATION
SECRET
lj OF ij PAGES
C5-3cii26
114
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Appendix 6
X'15 Flight Designation System
Appendix 6
X-15 Flight Designation System
The X-1 5 flight desig-
nation system used
for the vast majority of
the program was for-
malized in this 24 May
1 960 letter from Paul
Bikle.
May 24, i960
Prom NASA Flight Research Center
To NASA Headquarters RSS (Mr. H. Brown)
Subject: X-15 flight designation
1. At the suggestion of ARDC a system of flight desig-
nation for X-1 5 flight operations has been agreed upon by
NASA FRC, AFFTC, and HAA personnel. The system will cover
completed flights as well as planned flights; therefore, all
personnel concerned should use the flight-designation system
as soon as possible.
2. The flight-designation system consists of a three-
column designation. The first column indicates the X-15 air-
plane by number (l, 2, or 3). The second column indicates
the particular free-flight number of a given X-15, or whether
the mission was a planned captive flight (0) or an aborted
flight (A). The third column indicates the number of air-
borne X-15/B-52 missions for a given X-15- Designations of
flights to date are:
X-15-1 X-15-1 (Cont'd.) X-15-2 X-15-2 (Cont'd)
1-C-l
1-4-9
2-C-l
2-3-9-
l-A-2
1-5-10
2-A-2
2-A-10
l-A-3
1-6-11
2-1-3
2-A-4
2-4-11
l-A-4
1-7-12
2-5-12
1-1-5
1-8-13
2-A-5
2-6-13
2-A-14
l-A-6
2-2-6
1-2-7
1-3-8
2-A-7
2-A-8
2-8-16
2- A- 17
3. The designation of the next scheduled flights on all
X-15 airplanes will be 1-9-14 (X-15-0-), 2-9-18 (X-15-2), and
3-1-1 (X-15-3). ~f
:PauINF. Bikle
Director., NASA Flight Research Center
TWF:pm
TAT
DEB
Copies to:
NASA Ames Research Center (2)
NASA Langley Research Center
Attention: Mr. H. A. Soule'
(3)
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
115
Major Michael J. Adams Joins the Program
Appendix 7
Appendix 7
Major Michael J. Adams Joins the Program
DEPARTMENT OF THE AIR FORCE
HEADQUARTERS. AIR FORCE FLIGHT TEST CENTER (AFSC)
EDWARDS AIR FORCE BASE. CALIF. 93523
REPLY TO
ATTN OF: FTTO
subject: Selection of Crew Member for X-15 Program
14 JUL 1966
to: NASA (Mr. Bickle)
1. Major Michael J. Adams has been selected from a number of our
experimental test pilots to participate in the X-15 program. His
selection vas based on experience and past performance displayed
while assigned to the Air Force Plight Test Center. Major Adams
completed the Experimental Test Pilot Course (Class 62-c) and the
Aerospace Research Pilot Course, graduating number one in his class
from the Experimental Test Pilot Course. While assigned to the
Directorate of Flight Test Operations, he completed a variety of
test projects which included F-5A Category II Stability and Control
Tests and a longitudinal variable stability investigation to determine
optimum fighter aircraft characteristics. We believe Major Adams
has the ability and sound mature judgment required to adapt to the
rigors of a research program such as the X-15.
2. A brief resume of his military and flight experience follows:
Year
Assignment
1950
1951
1952
1952
1951*
1956
1958
1959
1962
1963
1963
1965
1951 Enlisted in USAF (Basic Training)
Link Trainer Instructor
1952 Aviation Cadet (Pilot Training)
Combat Crew Training (F-80/F-86)
1953 Fighter Pilot, 80th FBS (1*9
Combat Missions)
1956 Fighter Pilot, 6l3th FBS
1958 Student (B.S.A.E.)
1959 Student (G-rad Astronautics)
1962 Instructor (Maint Offr Course)
1963 Student (Exp Test Pilot Course 62-
Aerospace Research Pilot School
Class IV
1965 Experimental Test Pilot (Fighter
Branch)
1966 Crew Member (MOL)
Location
Lackland AFB, Texas
Reese AFB, Texas
Spencefield, Georgia
Webb AFB, Texas
Hellis AFB, Nevada
K-13, Suwon, Korea
England AFB, La.
Univ of Oklahoma
MIT, Cambridge, Mass.
Chanute AFB, 111.
C) Edwards AFB, Calif.
Edwards AFB, Calif.
Edwards AFB, Calif.
SSD, El Segundo, Calif.
Flying Experience:
Total Time
Single Engine Jet
Multi Jet
39^0:00
2505:00 (F-80/F-8UF/F-86/F-10tyF-106/T-33 primarily)
1*77:00 (F-5/T-38/F-101 primarily)
(/
3. As additional information, a photograph and brief biographic sketch are
included.
2 Atch
1. Photo
2. Biographical Sketch
CL^DE-ST CHERRY, ColeShel, USAF
iSiputy for Systes»<Test
Cy to:
ASD (ASZVE)
AFSC (SCSAN/Col lake)
Major Michael J.
Adams was assigned
to the X-1 5 program in
the summer of 1966,
coming straight from
the ill-fated Manned
Orbiting Laboratory
program.
Adams would make
six successful X-15
flights, but was killed
during a high altitude
flight on 15 November
1967.
116
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Appendix 8
Astronaut Wings
Appendix 8
Astronaut Wings
Over the years there
has been a great deal
of debate regarding if
theX-15 pilots were
"astronauts." By the
definitions in place at
the time, the Air Force
pilots that flew above
50 statue miles alti-
tude were awarded
Astronaut Wings.
Under these rules,
Adams, Engle, Knight,
Rushworth, and White
qualified.
The orders that
awarded Astronaut
Wings to the Air Force
pilots were nothing out
of the ordinary. A sim-
ple sheet of paper —
no certificate; not even
an embossed seal or
a real signature.
Michael Adams was
awarded his Astronaut
Wings posthumously
after he was killed on
his only flight above
50 miles. This copy of
his orders was largely
responsible for getting
Adams' name on the
Astronaut Memorial at
the Kennedy Space
Center, Florida.
DEPARTMENT OF THE AIR FORCE
WASHINGTON
AERONAUTICAL ORDER
40
22 April 1968
MAJ WILLIAM J KNIGHT, FR53263, AF Flight Test Center, AFSC,
Edwards AFB, Calif 93523, is awarded the aeronautical rating of
COMMAND PILOT ASTRONAUT per para 1-22, AFM 35-13. Authority:
Para 1-20, AFM 35-13.
BY ORDER OF THE SECRETARY OF THE AIR FORCE
J. P. McCONNELL, General, USAF
Chief of Staff
R. J. PUGH, Colonel, USAF
Director of Administrative Services
DEPARTMENT OF THE AIR FORCE
WASHINGTON
AERONAUTICAL ORDER
130
15 November 1967
MAJ MICHAEL J ADAMS, FR24934, AF Flight Test Center, AFSC,
Edwards AFB, Calif 93523, is awarded the aeronautical rating of
COMMAND PILOT ASTRONAUT per para 1-22, AFM 35-13.
Authority: Para 1-20, AFM 35-13.
BY ORDER OF THE SECRETARY OF THE AIR FORCE
J. P. McCONNELL, General, USAF
Chief pf Staff
R. J. PUGH, Colonel, USAF
Director of Administrative Services
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
117
X-15 Program Flight Log
Appendix 9
Appendix 9
X-15 Program Flight Log
Flight
Flight
Serial
Max.
Max.
Max.
No.
ID
No.
Date
Pilot
Mach
Altitude
Speed
1
1-1-5
56-6670
08 Jun 59
Crossfield
0.79
37,550
522
2
2-1-3
56-6671
17 Sep 59
Crossfield
2.11
52,341
1,393
3
2-2-6
56-6671
17 Oct 59
Crossfield
2.15
61,781
1,419
4
2-3-9
56-6671
05 Nov 59
Crossfield
1.00
45,462
660
5
1-2-7
56-6670
23 Jan 60
Crossfield
2.53
66,844
1,669
6
2-4-11
56-6671
11 Feb 60
Crossfield
2.22
88,116
1,466
7
2-5-12
56-6671
17 Feb 60
Crossfield
1.57
52,640
1,036
8
2-6-13
56-6671
17 Mar 60
Crossfield
2.15
52,640
1,419
9
1-3-8
56-6670
25 Mar 60
Walker
2.00
48,630
1,320
10
2-7-15
56-6671
29 Mar 60
Crossfield
1.96
49,982
1,293
11
2-8-16
56-6671
31 Mar 60
Crossfield
2.03
51,356
1,340
12
1-4-9
56-6670
13 Apr 60
White
1.90
48,000
1,254
13
1-5-10
56-6670
19 Apr 60
Walker
2.56
59,496
1,689
14
1-6-11
56-6670
06 May 60
White
2.20
60,938
1,452
15
1-7-12
56-6670
12 May 60
Walker
3.19
77,882
2,111
16
1-8-13
56-6670
19 May 60
White
2.31
108,997
1,590
17
2-9-18
56-6671
26 May 60
Crossfield
2.20
51,282
1,452
18
1-9-17
56-6670
04 Aug 60
Walker
3.31
78,112
2,195
19
1-10-19
56-6670
12 Aug 60
White
2.52
136,500
1,772
20
1-11-21
56-6670
19 Aug 60
Walker
3.13
75,982
1,986
21
1-12-23
56-6670
10 Sep 60
White
3.23
79,864
2,182
22
1-13-25
56-6670
23 Sep 60
Petersen
1.68
53,043
1,108
23
1-14-27
56-6670
20 Oct 60
Petersen
1.94
53,800
1,280
24
1-15-28
56-6670
28 Oct 60
McKay
2.02
50,700
1,333
25
146-29
56-6670
04 Nov 60
Rushworth
1.95
48,900
1,287
26
2-10-21
56-6671
15 Nov 60
Crossfield
2.97
81,200
1,960
27
1-17-30
56-6670
17 Nov 60
Rushworth
1.90
54,750
1,254
28
2-11-22
56-6671
22 Nov 60
Crossfield
2.51
61,900
1,656
29
1-18-31
56-6670
30 Nov 60
Armstrong
1.75
48,840
1,155
30
2-12-23
56-6671
06 Dec 60
Crossfield
2.85
53,374
1,881
31
1-19-32
56-6670
09 Dec 60
Armstrong
1.80
50,095
1,188
32
1-20-35
56-6670
01 Feb 61
McKay
1.88
49,780
1,211
33
1-21-36
56-6670
07 Feb 61
White
3.50
78,150
2,275
34
2-13-26
56-6671
07 Mar 61
White
4.43
77,450
2,905
35
2-14-28
56-6671
30 Mar 61
Walker
3.95
169,600
2,760
36
2-15-29
56-6671
21 Apr 61
White
4.62
105,000
3,074
37
2-16-31
56-6671
25 May 61
Walker
4.95
107,500
3,307
38
2-17-33
56-6671
23 Jun 61
White
5.27
107,700
3,603
39
1-22-37
56-6670
10 Aug 61
Petersen
4.11
78,200
2,735
40
2-18-34
56-6671
12 Sep 61
Walker
5.21
114,300
3,618
41
2-19-35
56-6671
28 Sep 61
Petersen
5.30
101,800
3,600
42
1-23-39
56-6670
04 Oct 61
Rushworth
4.30
78,000
2,830
43
2-20-36
56-6671
11 Oct 61
White
5.21
217,000
3,647
Twelve pilots flew the
X-15. Scott Crossfield
was first. William Dana
was last. Pete Knight
went more than 4,500
miles per hour. Joe
Walker went more
than 67 miles high.
Michael Adams died.
The X-1 5 program is
arguably the most
successful flight
research program ever
undertaken by the
United States. The
1 99 flights made by
the three research air-
planes contributed not
only to aeronautical
science, but provided
many answers the
United States needed
to get to the Moon
during Project Apollo.
Flight number 38 rep-
resented the first
Mach 5 flight made by
any manned
aircraft.
118
H?f>eTsonics Before the Shuttle — Monographs in Aerospace History 'Number 1 8
Appendix 9
X-15 Program Flight Log
Flight number 45 rep-
resented the first
Mach 6 flight made by
any manned
aircraft.
Flight number 46 was
the first flight for the
third X-15.
Flight number 52 set
an FAI certified alti-
tude record.
Flight number 53 was
the first flight with a
dynamic pressure
over 2,000 psf.
Flight number 62 set
another FAI certified
altitude record for
class.
Flight number 91 was
the highest X-15 flight;
354,200 feet— almost
67 miles high
Flight
Flight
Serial
Max.
Max.
Max.
No.
ID
No.
Date
Pilot
Mach
Altitude
Speed
44
1-24-40
56-6670
17 Oct 61
Walker
5.74
108,600
3,900
45
2-21-37
56-6671
09 Nov 61
White
6.04
101,600
4,093
46
3-1-2
56-6672
20 Dec 61
Armstrong
3.76
81,000
2,502
47
1-25-44
56-6670
10 Jan 62
Petersen
0.97
44,750
645
48
3-2-3
56-6672
17 Jan 62
Armstrong
5.51
133,500
3,765
49
3-3-7
56-6672
05 Apr 62
Armstrong
4.12
180,000
2,850
50
1-26-46
56-6670
19 Apr 62
Walker
5.69
154,000
3,866
51
3-4-8
56-6672
20 Apr 62
Armstrong
5.31
207,500
3,789
52
1-27-48
56-6670
30 Apr 62
Walker
4.94
246,700
3,489
53
2-22-40
56-6671
08 May 62
Rushworth
5.34
70,400
3,524
54
1-28-49
56-6670
22 May 62
Rushworth
5.03
100,400
3,450
55
2-23-43
56-6671
01 Jun 62
White
5.42
132,600
3,675
56
1-29-50
56-6670
07 Jun 62
Walker
5.39
103,600
3,672
57
3-5-9
56-6672
12 Jun 62
White
5.02
184,600
3,517
58
3-6-10
56-6672
21 Jun 62
White
5.08
246,700
3,641
59
1-30-51
56-6670
27 Jun 62
Walker
5.92
123,700
4,104
60
2-24-44
56-6671
29 Jun 62
McKay
4.95
83,200
3,280
61
1-31-52
56-6670
16 Jul 62
Walker
5.37
107,200
3,674
62
3-7-14
56-6672
17 Jul 62
White
5.45
314,750
3,832
63
2-25-45
56-6671
19 Jul 62
McKay
5.18
85,250
3,474
64
1-32-53
56-6670
26 Jul 62
Armstrong
5.74
98,900
3,989
65
3-8-16
56-6672
02 Aug 62
Walker
5.07
144,500
3,438
66
2-26-46
56-6671
08 Aug 62
Rushworth
4.40
90,877
2,943
67
3-9-18
56-6672
14 Aug 62
Walker
5.25
193,600
3,747
68
2-27-47
56-6671
20 Aug 62
Rushworth
5.24
88,900
3,534
69
2-28-48
56-6671
29 Aug 62
Rushworth
5.12
97,200
3,447
70
2-29-50
56-6671
28 Sep 62
McKay
4.22
68,200
2,765
71
3-10-19
56-6672
04 Oct 62
Rushworth
5.17
112,200
3,493
72
2-30-51
56-6671
09 Oct 62
McKay
5.46
130,200
3,716
73
3-11-20
56-6672
23 Oct 62
Rushworth
5.47
134,500
3,716
74
2-31-52
56-6671
09 Nov 62
McKay
1.49
53,950
1,019
75
3-12-22
56-6672
14 Dec 62
White
5.65
141,400
3,742
76
3-13-23
56-6672
20 Dec 62
Walker
5.73
160,400
3,793
77
3-14-24
56-6672
17 Jan 63
Walker
5.47
271,700
3,677
78
1-33-54
56-6670
11 Apr 63
Rushworth
4.25
74,400
2,864
79
3-15-25
56-6672
18 Apr 63
Walker
5.51
92,500
3,770
80
1-34-55
56-6670
25 Apr 63
McKay
5.32
105,500
3,654
81
3-16-26
56-6672
02 May 63
Walker
4.73
209,400
3,488
82
3-17-28
56-6672
14 May 63
Rushworth
5.20
95,600
3,600
83
1-35-56
56-6670
15 May 63
McKay
5.57
124,200
3,856
84
3-18-29
56-6672
29 May 63
Walker
5.52
92,000
3,858
85
3-19-30
56-6672
18 Jun 63
Rushworth
4.97
223,700
3,539
86
1-36-57
56-6670
25 Jun 63
Walker
5.51
111,800
3,911
87
3-20-31
56-6672
27 Jun 63
Rushworth
4.89
285,000
3,425
88
1-37-59
56-6670
09 Jul 63
Walker
5.07
226,400
3,631
89
1-38-61
56-6670
18 Jul 63
Rushworth
5.63
104,800
3,925
90
3-21-32
56-6672
19 Jul 63
Walker
5.50
347,800
3,710
91
3-22-36
56-6672
22 Aug 63
Walker
5.58
354,200
3,794
92
1-39-63
56-6670
07 Oct 63
Engle
4.21
77,800
2,834
93
1-40-64
56-6670
29 Oct 63
Thompson
4.10
74,400
2,712
94
3-23-39
56-6672
07 Nov 63
Rushworth
4.40
82,300
2,925
95
1-41-65
56-6670
14 Nov 63
Engle
4.75
90,800
3,286
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
119
X'15 Program Flight Log
Appendix 9
Flight
Flight
Serial
Max.
Max.
Max.
No.
ID
No.
Date
Pilot
Mach
Altitude
Speed
96
3-24-41
56-6672
27 Nov 63
Thompson
4.94
89,800
3,310
97
1-42-67
56-6670
05 Dec 63
Rushworth
6.06
101,000
4,018
98
1-43-69
56-6670
08 Jan 64
Engle
5.32
139,900
3,616
99
3-25-42
56-6672
16 Jan 64
Thompson
4.92
71,000
3,242
100
1-44-70
56-6670
28 Jan 64
Rushworth
5.34
107,400
3,618
101
3-26-43
56-6672
19 Feb 64
Thompson
5.29
78,600
3,519
102
3-27-44
56-6672
13 Mar 64
McKay
5.11
76,000
3,392
103
1-45-72
56-6670
27 Mar 64
Rushworth
5.63
101,500
3,827
104
1-46-73
56-6670
08 Apr 64
Engle
5.01
175,000
3,468
105
1-47-74
56-6670
29 Apr 64
Rushworth
5.72
101,600
3,906
106
3-28-47
56-6672
12 May 64
McKay
4.66
72,800
3,084
107
1-48-75
56-6670
19 May 64
Engle
5.02
195,800
3,494
108
3-29-48
56-6672
21 May 64
Thompson
2.90
64,200
1,865
109
2-32-55
56-6671
25 Jun 64
Rushworth
4.59
83,300
3,104
110
1-49-77
56-6670
30 Jun 64
McKay
4.96
99,600
3,334
111
3-30-50
56-6672
08 Jul 64
Engle
5.05
170,400
3,520
112
3-31-52
56-6672
29 Jul 64
Engle
5.38
78,000
3,623
113
3-32-53
56-6672
12 Aug 64
Thompson
5.24
81,200
3,535
114
2-33-56
56-6671
14 Aug 64
Rushworth
5.23
103,300
3,590
115
3-33-54
56-6672
26 Aug 64
McKay
5.65
91,000
3,863
116
3-34-55
56-6672
03 Sep 64
Thompson
5.35
78,600
3,615
117
3-35-57
56-6672
28 Sep 64
Engle
5.59
97,000
3,888
118
2-34-57
56-6671
29 Sep 64
Rushworth
5.20
97,800
3,542
119
1-50-79
56-6670
15 Oct 64
McKay
4.56
84,900
3,048
120
3-36-59
56-6672
30 Oct 64
Thompson
4.66
84,600
3,113
121
2-35-60
56-6671
30 Nov 64
McKay
4.66
87,200
3,089
122
3-37-60
56-6672
09 Dec 64
Thompson
5.42
92,400
3,723
123
1-51-81
56-6670
10 Dec 64
Engle
5.35
113,200
3,675
124
3-38-61
56-6672
22 Dec 64
Rushworth
5.55
81,200
3,593
125
3-39-62
56-6672
13 Jan 65
Thompson
5.48
99,400
3,712
126
3-40-63
56-6672
02 Feb 65
Engle
5.71
98,200
3,885
127
2-36-63
56-6671
17 Feb 65
Rushworth
5.27
95,100
3,539
128
1-52-85
56-6670
26 Feb 65
McKay
5.40
153,600
3,702
129
1-53-86
56-6670
26 Mar 65
Rushworth
5.17
101,900
3,580
130
3-41-64
56-6672
23 Apr 65
Engle
5.48
79,700
3,657
131
2-37-64
56-6671
28 Apr 65
McKay
4.80
92,600
3,260
132
2-38-66
56-6671
18 May 65
McKay
5.17
102,100
3,541
133
1-54-88
56-6670
25 May 65
Thompson
4.87
179,800
3,418
134
3-42-65
56-6672
28 May 65
Engle
5.17
209,600
3,754
135
3-43-66
56-6672
16 Jun 65
Engle
4.69
244,700
3,404
136
1-55-89
56-6670
17 Jun 65
Thompson
5.14
108,500
3,541
137
2-39-70
56-6671
22 Jun 65
McKay
5.64
155,900
3,938
138
3-44-67
56-6672
29 Jun 65
Engle
4.94
280,600
3,432
139
2-40-72
56-6671
08 Jul 65
McKay
5.19
212,600
3,659
140
3-45-69
56-6672
20 Jul 65
Rushworth
5.40
105,400
3,760
141
2-41-73
56-6671
03 Aug 65
Rushworth
5.16
208,700
3,602
142
1-56-93
56-6670
06 Aug 65
Thompson
5.15
103,200
3,534
143
3-46-70
56-6672
10 Aug 65
Engle
5.20
271,000
3,550
144
1-57-96
56-6670
25 Aug 65
Thompson
5.11
214,100
3,604
145
3-47-71
56-6672
26 Aug 65
Rushworth
4.79
239,600
3,372
146
2-42-74
56-6671
02 Sep 65
McKay
5.16
239,800
3,570
147
1-58-97
56-6670
09 Sep 65
Rushworth
5.25
97,200
3,534
Flight number 109
was the first flight of
the modified
X-15A-2.
Flight number 114
had the nose gear
inadvertently extend
at Mach 4.2.
Flight number 119
was the first flight with
wing-tip pods
installed.
Flight number 127 had
the right main skid
extend inadvertently at
Mach 4.3 and 85,000
feet.
Flight number 131
flew with the damper
(augmentation) off at
a dynamic pressure of
1 ,500 psf; the highest
of the program.
120
Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Appendix 9
X-1 5 Program Flight Log
Flight number 155
was the first flight with
(empty) external
tanks.
Flight number 166
recorded the highest
dynamic pressure of
any X-1 5 flight; 2,202
psf.
Flight number 186
was the first flight with
full ablative coating.
No tanks.
Flight number 188
was the fastest flight
of the X-1 5 program;
4,520 mph.
Flight number 191
resulted in the death
of Major Michael J.
Adams; the only
fatality during the
X-1 5 program
Flight
Flight
Serial
Max.
Max.
Max.
No.
ID
No.
Date
Pilot
Mach
Altitude
Speed
148
3-48-72
56-6672
14 Sep 65
McKay
5.03
239,000
3,519
149
1-59-98
56-6670
22 Sep 65
Rushworth
5.18
100,300
3,550
150
3-49-73
56-6672
28 Sep 65
McKay
5.33
295,600
3,732
151
1-60-99
56-6670
30 Sep 65
Knight
4.06
76,600
2,718
152
3-50-74
56-6672
12 Oct 65
Knight
4.62
94,400
3,108
153
1-61-101
56-6670
14 Oct 65
Engle
5.08
266,500
3,554
154
3-51-75
56-6672
27 Oct 65
McKay
5.06
236,900
3,519
155
2-43-75
56-6671
03 Nov 65
Rushworth
2.31
70,600
1,500
156
1-62-103
56-6670
04 Nov 65
Dana
4.22
80,200
2,765
157
1-63-104
56-6670
06 May 66
McKay
2.21
68,400
1,434
158
2-44-79
56-6671
18 May 66
Rushworth
5.43
99,000
3,689
159
2-45-81
56-6671
01 Jul 66
Rushworth
1.70
44,800
1,061
160
1-64-107
56-6670
12 Jul 66
Knight
5.34
130,000
3,661
161
3-52-78
56-6672
18 Jul 66
Dana
4.71
96,100
3,217
162
2-46-83
56-6671
21 Jul 66
Knight
5.12
192,300
3,568
163
1-65-108
56-6670
28 Jul 66
McKay
5.19
241,800
3,702
164
2-47-84
56-6671
03 Aug 66
Knight
5.03
249,000
3,440
165
3-53-79
56-6672
04 Aug 66
Dana
5.34
132,700
3,693
166
1-66-111
56-6670
11 Aug 66
McKay
5.21
251,000
3,590
167
2-48-85
56-6671
12 Aug 66
Knight
5.02
231,100
3,472
168
3-54-80
56-6672
19 Aug 66
Dana
5.20
178,000
3,607
169
1-67-112
56-6670
25 Aug 66
McKay
5.11
257,500
3,543
170
2-49-86
56-6671
30 Aug 66
Knight
5.21
100,200
3,543
171
1-68-113
56-6670
08 Sep 66
McKay
2.44
73,200
1,602
172
3-55-82
56-6672
14 Sep 66
Dana
5.12
254,200
3,586
173
1-69-116
56-6670
06 Oct 66
Adams
3.00
75,400
1,977
174
3-56-83
56-6672
01 Nov 66
Dana
5.46
306,900
3,750
175
2-50-89
56-6671
18 Nov 66
Knight
6.33
98,900
4,250
176
3-57-86
56-6672
29 Nov 66
Adams
4.65
92,000
3,120
177
1-70-119
56-6670
22 Mar 67
Adams
5.59
133,100
3,822
178
3-58-87
56-6672
26 Apr 67
Dana
1.80
53,400
1,163
179
1-71-121
56-6670
28 Apr 67
Adams
5.44
167,200
3,720
180
2-51-92
56-6671
08 May 67
Knight
4.75
97,600
3,193
181
3-59-89
56-6672
17 May 67
Dana
4.80
71,100
3,177
182
1-72-125
56-6670
15 Jun 67
Adams
5.14
229,300
3,606
183
3-60-90
56-6672
22 Jun 67
Dana
5.34
82,200
3,611
184
1-73-126
56-6670
29 Jun 67
Knight
4.17
173,000
2,870
185
3-61-91
56-6672
20 Jul 67
Dana
5.44
84,300
3,693
186
2-52-96
56-6671
21 Aug 67
Knight
4.94
91,000
3,368
187
3-62-92
56-6672
25 Aug 67
Adams
4.63
84,400
3,115
188
2-53-97
56-6671
03 Oct 67
Knight
6.70
102,100
4,520
189
3-63-94
56-6672
04 Oct 67
Dana
5.53
251,100
3,897
190
3-64-95
56-6672
17 Oct 67
Knight
5.53
280,500
3,869
191
3-65-97
56-6672
15 Nov 67
Adams
5.20
266,000
3,617
192
1-74-130
56-6670
01 Mar 68
Dana
4.36
104,500
2,878
193
1-75-133
56-6670
04 Apr 68
Dana
5.27
187,500
3,610
194
1-76-134
56-6670
26 Apr 68
Knight
5.05
209,600
3,545
195
1-77-136
56-6670
12 Jun 68
Dana
5.15
220,100
3,563
196
1-78-138
56-6670
16 Jul 68
Knight
4.79
221,500
3,382
197
1-79-139
56-6670
21 Aug 68
Dana
5.01
267,500
3,443
198
1-80-140
56-6670
13 Sep 68
Knight
5.37
254,100
3,723
199
1-81-141
56-6670
24 Oct 68
Dana
5.38
255,000
3,716
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle
121
Index
Index
A3 Hermes rocket engine, 13
A/P22S pressure suit, 39, 53, 70
See also MC-2 pressure suit
ablative coatings, 58-60, 68, 79
accidents, 30, 48, 50, 59, 61, 80, 81
Accident Board, 62
Mike Adams death, 62, 77, 81
Adams, Michael J., Major, 61-62, 67
Accident. See Accidents
adaptive control systems
See MH-96
Aerojet General, 16
AF33(600)-31693 (X-15 contract), 18
AF33(600)-32248 (XLR99 contract), 18
Air Force
See U. S. Air Force
Air Force Flight Test Center, 15, 32, 42, 45, 73
Ames Aeronautical Laboratory, 17
10-by-14 inch supersonic tunnel, 16
Hypervelocity free-flight facility, 25
X-15 proposal evaluation, 17
ammonia, use as propellant, 35
Apt, Milhum G., Captain, 9, 40
Armstrong, Neil A., 23
Astronaut Memorial, 62
Astronaut Wings, 61,67
asymmetrical heating, 13
auxiliary power unit, 38, 42
APU problems, 47-51,56
B
B-36, Convair, 9, 18, 33, 41
B-50, Boeing, 42
B-52, Boeing, 37, 40, 42, 45, 47, 55, 61
B.F. Goodrich Company, 70
ball-nose, 29, 51, 76
Ballarat Dry Lake, 62
ballistic controls, 10,28,32,51,77
Beatty, Nevada, 41
122 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Index
Becker, John V., 8, 1 1-12, 54, 60, 67, 68, 80
Beeler, De E., 14
Bell Aircraft, 12,15-17
Bellman, Donald R., 62
Bikle, Paul R, 55
Boeing Company, 15, 16
See also B-50, Boeing
See also B-52, Boeing
Boston, Ronald G., 67
Bredt, Irene, 7
Brown, Clinton E., 7
Buckley, Edmond C, 73, 74
cancellation of X-15 flight program, 63
captive-carry flights, 47
Carl, Marion, 70
carrier aircraft
See B-52, Boeing
See also B-36, Convair
Chance-Vought, 15, 16
Conference on the Progress of the X-15 Project, 24, 42
Convair (Consolidated Vultee), 15,16
Crossfield, A. Scott, 14, 21-22, 29, 39, 47-51, 67
first X-15 flight, 48
first flight with XLR99, 52
last X-15 flight, 52
Cuddeback Dry Lake, 62
D
Dana, William H., 53, 59, 63, 67, 80
David Clark Company, 39, 70
Delamar Dry Lake, 61
Douglas Aircraft Company, 13,15-17
Dow, Norris E, 8
Dryden, Hugh L., Dr., 13, 15, 71
dummy ramjet
See Hypersonic Research Engine
Dyna-Soar, 56
E
ejection system, 22, 29-30, 39-40
Electronic Engineering Company, 41
Ely, Nevada, 41
Engle, Joseph H„ Captain, 61
external fuel tanks, 57
Monographs in Aerospace History Number 18 — Hy personics Before the Shuttle 123
Index
Faget, Maxim A., 8
Feltz, Charles H., 21
first government X-15 flight, 50
Flight Research Center, 22, 50, 53-54, 62, 72, 79
See also High-Speed Flight Station
follow-on experiments, 77
See also Hypersonic Research Engine
Freeman, E. C, Major, 32
fuselage side tunnels, 24, 27
G
Gardner, Trevor, 14
Garrett-AirResearch, 79
General Electric, 13, 38
Gilruth, Robert R., 7
Goldin, Daniel, 81
Greene, Lawrence R, 31
Grumman Aircraft Corporation, 15, 16
H
Haugen, V. R., Colonel, 14
heating projections, 1 1
Hedgepefh, John, 42
Hermes rocket engine
See A3 Hermes rocket engine
High Range, 41, 56, 73, 81
high-lift, 10
High-Speed Flight Station, 14, 16, 42, 45, 70
X-15 proposal evaluation, 17
See also Flight Research Center
hot-structure, 12, 26-27, 58, 60, 75
Hunley, J. D. "Dill," 67
Hypersonic Research Engine, 57, 62, 79, 80
dummy ramjet, 58-60
Inconel X, 12, 23, 26-27, 29, 31, 54, 58, 75, 79
Intercontinental Ballistic Missile, 8
K
Kelset, Benjamin S., Brigadier General, 14
Kincheloe, Iven C, Captain, 23, 32, 33
Knight, William J. "Pete," Major, 58-59, 61, 63, 67
Kolf, Jack, 60
124 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Index
Langley Aeronautical Laboratory, 7-11, 13, 15, 41, 50, 79
1 1-inch hypersonic wind tunnel, 8
Aero-Physics Division, 8
Instrument Research Division, 73
Mach 4 blowdown tunnel, 16
Tracking and Ground Instrumentation Group, 73
X-15 proposal evaluation, 17
last X-15 flight, 63
lessons learned, 67
Lewis Research Center, 35
Lindell, Keith G., Lieutenant Colonel, 32
Lockheed Aircraft Company, 15
low-L/D, 10
M
MA-25S coating, 58
See also ablative coatings
Martin Company, 15, 58
Matay, Doll, 42
MC-2 pressure suit, 39, 53
See also A/P22S pressure suit
McCollough, Chester E., Jr., 32
McDonnell Aircraft Company, 15-16
McKay, John B. "Jack," 23, 57
McLellan, Charles H., 8, 11
Memorandum of Understanding, 14
MH-96 adaptive control system, 61-62
Millikan, Clarke, 7
Minneapolis Honeywell, 61, 77
mockup inspection, 32
modifications (X-15A-2), 57
first flight of X- 15 A-2, 58
mothership
See B-52, Boeing
See also B-36, Convair
iV
NA-5400, North American, 16
NASA 1 Control Room, 41, 59, 61, 74
National Advisory Committee for Aeronautics (NACA), 7
Ames Aeronautical Laboratory
See Ames Aeronautical Laboratory
becomes NASA, 42
Committee on Aerodynamics, 7
Committee on Aeronautics, 14
Executive Committee, 7
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 125
Index
Headquarters, 8
Langley Aeronautical Laboratory
See Langley Aeronautical Laboratory
Research Airplane Panel, 8
X-15 proposal evaluation, 16-17
National Aeronautics and Space Administration (NASA), 42
Navy
See U.S. Navy
NB-52. See B-52, Boeing
North American Aviation, 15-18, 22-24, 31, 33-34, 38-40, 45, 50, 57, 68
notified as X-15 winner, 18
withdrawal of X- 1 5 proposal, 1 7
X-15 contract, 18
Northrop Aircraft Company, 15, 16
o
O'Sullivan, William J., Jr., 7
Orazio, R, 32
Orbiting Astronomical Observatory, 57
physiological effects, 31,69
Plasmakote Corporation, 56
pressure suits, 39
A/P22S. See A/P22S pressure suit
MC-2. See MC-2 pressure suit
Project 1226, 18
Project Mercury, 55, 69
Putt, Donald, Lieutenant General, 13
R
reaction controls
See ballistic controls
Reaction Motors, Inc., 16, 34- 37
XLR99 contract, 18
Republic Aviation Corporation, 15-17
Research Airplane Committee, 15
Reynolds numbers, 9
Rice, Raymond H., 17-18
Rogers Dry Lake, 62
roll instability, 54
roll out, 42, 46
rolling-tail, 23, 26, 75
Rosamond Dry Lake, 49
Rushworth, Robert A., Lieutenant Colonel, 58
126 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
Index
Sanger, Eugen, 7
Schoech, W. A., Rear Admiral, 34
side-stick controller, 28, 70, 77
Smith, J. H., Jr., 15
Soule, Hartley A., 8, 16, 21
Space Shuttle, 11, 55, 58, 59, 60, 61, 68, 69, 71, 73, 74, 75, 76, 79, 81
Space Task Group, 74
Spaulding-Chi model, 71
Sperry Gyroscope Company, 40, 56
stable-platform, 40
Stewart, James T., Brigadier General, 63
Storms, Harrison A. "Stormy," Jr., 21, 68
Toll, Thomas A., 8
Truszynski, Gerald M., 73, 74
u
U. S. Air Force, 7
AFFTC. See Air Force Flight Test Center
Aero Medical Laboratory, 39
Aeronautical Systems Division, 56
Air Research and Development Command, 14, 32
Headquarters, 13
Materials Laboratory, 56
Scientific Advisory Board, 7, 13
Wright Air Development Center, 13, 14, 32, 37
Power Plant Laboratory, 33, 34
Wright Field, 15-16,39
X-15 proposal evaluation, 17
U.S. Navy, 7
Bureau of Aeronautics (BuAer), 7,34
NADC Johnsville, 23, 72
Office of Naval Research, 13
X-15 proposal evaluation, 1 7
w
Walker, Joseph A., 23, 32, 53, 61, 67
first X-15 flight, 50
Webb, James, 74
wedge shape tail, 1 1, 72
Wendover, Utah, 41
White, Alvin M., 23
White, Robert M., Major, 23, 50, 52, 53
Whitten, James B., 8
Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 127
Index
Williams, Walter C, 14, 74
windshields fractured, 54
world speed record, 59
World WarH, 7,38
Wright Field, See U.S. Air Force; Wright Field
X-l, Bell, 8, 10, 26, 41, 42, 45, 77
X-2, BeU, 9, 10, 14, 26, 40, 41, 42
X-15, North American
contract, 18
delivery date for, 18
first flight, 48
first flight of X-15A-2, 58
first flight with XLR99, 52
flight designation system, 45
last flight, 63
modifications to X-15A-2, 57
officially designated, 16, 45
serial numbers, 24, 45
X-24C National Hypersonic Flight Research Facility Program, 67
XLR8, Reaction Motors, 16
XLR10, Reaction Motors, 16, 34
XLR1 1, Reaction Motors, 16, 37, 50, 52
APU problems, 53
first XLR1 1 flight in X-15, 48, 50
last XLR1 1 flight in X-15, 52
XLR25, Curtiss-Wright, 16
XLR30, Reaction Motors, 16, 34
XLR73, Aerojet, 16
XLR81, Bell, 16
XLR99, Reaction Motors, 27, 33, 35-38, 51-52, 55, 57, 69, 81
contract, 18
first XLR99 flight in X- 15, 50
Flight Rating Test, 37
PreUminary Flight Rating Test, 37
Zimmerman, Charles H., 7
128 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
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